Okay, I tried this again using the same code you are using and it looks like this is just a limitation of DotNet. It can't cast an object decimal directly into a double. 

It looks like you will have to cast to a decimal first.This works:

decimal d = (Decimal)e.Cell.Value;
if ((double)d > 0)
{
}

Or another option would be to try to parse the value from a string:

            double d;
            bool success = double.TryParse(e.Cell.Value.ToString(), out d);
            if (success && d > 0)
            {

Also , e.Cell.Value.GetType() == System.Decimal

The field in the database is Decimal(18,2)

Line of Code in AfterCellUpdate event.

if

((double)e.Cell.Value < 0)

System.InvalidCastException was unhandled by user code
  Message="Specified cast is not valid."
  Source="UGTest"
  StackTrace:
       at Mitsic.frmcEquipment.ugEquip_AfterCellUpdate(Object sender, CellEventArgs e) in C:\Documents and Settings\rand\My Documents\Visual Studio 2008\Projects\UGTest\UGTest\frmcUgTestt.cs:line 7532
       at Infragistics.Win.UltraWinGrid.UltraGrid.OnAfterCellUpdate(CellEventArgs e)
       at Infragistics.Win.UltraWinGrid.UltraGrid.FireEvent(GridEventIds id, EventArgs e)
       at Infragistics.Win.UltraWinGrid.UltraGridCell.SetValueInternal(Object value, Boolean suppressErrorMessagePrompt, Boolean fireInitializeRow, Boolean throwExceptionOnError, Boolean fireDataChanged)
       at Infragistics.Win.UltraWinGrid.UltraGridCell.SetValueInternal(Object value, Boolean suppressErrorMessagePrompt, Boolean fireInitializeRow, Boolean throwExceptionOnError)
       at Infragistics.Win.UltraWinGrid.UltraGridCell.SetValueInternal(Object value, Boolean suppressErrorMessagePrompt, Boolean fireInitializeRow)
       at Infragistics.Win.UltraWinGrid.UltraGridCell.CommitEditValue(Boolean& stayInEdit, Boolean fireDataError, Boolean forceDontThrowException, Boolean dontStoreInUndoHistory)
  InnerException: