Two lines, to re-assign a data source to a xamDataGrid (we do this a lot)

grid.DataSource = null;
grid.DataSource = rows;  // rows is IList<>

First pass, second line generates:

System.InvalidOperationException occurred
  Message="Can't set the DataSource on a DataPresenter that has items added explicitly through the DataItems collection."
  Source="Infragistics3.Wpf.DataPresenter.v9.1"
  StackTrace:
       at Infragistics.Windows.DataPresenter.DataPresenterBase.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at Infragistics.Windows.DataPresenter.XamDataGrid.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
       at System.Windows.DependencyObject.SetValue(DependencyPropertyKey key, Object value)
       at Infragistics.Windows.DataPresenter.DataPresenterBase.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at Infragistics.Windows.DataPresenter.XamDataGrid.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
       at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
       at Infragistics.Windows.DataPresenter.DataPresenterBase.set_DataSource(IEnumerable value)
       at GED.Frameworks.EnerPrise.UserInterface.ScenarioInspector.SetGridDataSource(IList`1 rows) in C:\Enerprise\Dev\Analytics0109\Source\Shared Components\User Interface Legacy\ScenarioInspector\ScenarioInspector.xaml.cs:line 450
  InnerException:

Second pass, first line generates:

System.ArgumentException was unhandled
  Message="DefaultFieldLayout can only be set to a FieldLayout contained in the FieldLayouts collection."
  Source="Infragistics3.Wpf.DataPresenter.v9.1"
  StackTrace:
       at Infragistics.Windows.DataPresenter.DataPresenterBase.set_DefaultFieldLayout(FieldLayout value)
       at Infragistics.Windows.DataPresenter.FieldLayout.set_IsDefault(Boolean value)
       at Infragistics.Windows.DataPresenter.DataPresenterBase.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at Infragistics.Windows.DataPresenter.XamDataGrid.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
       at System.Windows.DependencyObject.SetValue(DependencyPropertyKey key, Object value)
       at Infragistics.Windows.DataPresenter.DataPresenterBase.VerifyRecordManagerDataSource()
       at Infragistics.Windows.DataPresenter.DataPresenterBase.InitializeRecordManagerDataSource()
       at Infragistics.Windows.DataPresenter.DataPresenterBase.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at Infragistics.Windows.DataPresenter.XamDataGrid.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
       at System.Windows.DependencyObject.SetValue(DependencyPropertyKey key, Object value)
       at Infragistics.Windows.DataPresenter.DataPresenterBase.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at Infragistics.Windows.DataPresenter.XamDataGrid.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
       at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
       at Infragistics.Windows.DataPresenter.DataPresenterBase.set_DataSource(IEnumerable value)
       at GED.Frameworks.EnerPrise.UserInterface.ScenarioInspector.SetGridDataSource(IList`1 rows) in C:\Enerprise\Dev\Analytics0109\Source\Shared Components\User Interface Legacy\ScenarioInspector\ScenarioInspector.xaml.cs:line 449
       at GED.Frameworks.EnerPrise.UserInterface.ScenarioInspector.SetGridDataSource(InspectorData inspectorData) in C:\Enerprise\Dev\Analytics0109\Source\Shared Components\User Interface Legacy\ScenarioInspector\ScenarioInspector.xaml.cs:line 444
       at GED.Frameworks.EnerPrise.UserInterface.ScenarioInspector.Model_LoadingFinished(Object sender, LoadingCompleteEventArgs data) in C:\Enerprise\Dev\Analytics0109\Source\Shared Components\User Interface Legacy\ScenarioInspector\ScenarioInspector.xaml.cs:line 260
       at GED.Frameworks.EnerPrise.UserInterface.InspectorModel.LoadComplete(Object sender, RunWorkerCompletedEventArgs e) in C:\Enerprise\Dev\Analytics0109\Source\Shared Components\User Interface Legacy\ScenarioInspector\InspectorModel.cs:line 610
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
       at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
       at System.Windows.Threading.DispatcherOperation.InvokeImpl()
       at System.Threading.ExecutionContext.runTryCode(Object userData)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.ProcessQueue()
       at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
       at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
       at System.Windows.Threading.Dispatcher.TranslateAndDispatchMessage(MSG& msg)
       at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
       at System.Windows.Application.RunInternal(Window window)
       at Henwood.Shared.Enerprise.API.Dashboard.App.Main() in C:\Enerprise\Dev\Analytics0109\Source\Applications\Enerprise.Dashboard\obj\Debug\App.g.cs:line 0
       at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException:

Neither of these ever occurred in 2008.3. Any thoughts? We love (nay, NEED) the new features, but we're dead in the water right now.