Try this:

Create a cutom Field which his editor is XamMasketTexbox. 

then assume that f is an object of typeField and configure it:

                    f.DataType = typeof(string);
                    f.Settings.EditAsType = typeof(string);
                    f.Settings.EditorType = typeof(XamMaskedEditor);

                    Style s = new Style(typeof(XamMaskedEditor));
                    //Setter setterMask = new Setter(XamMaskedEditor.MaskProperty, mask);
                    Setter setterMask = new Setter(XamMaskedEditor.MaskProperty, "{char:10:0-9a-zA-Z)");
                    s.Setters.Add(setterMask);
                    f.Settings.EditorStyle = s;
 

 If you need a list of rules to generarte masks follow this link:

http://help.infragistics.com/Help/NetAdvantage/WPF/2007.2/CLR3.X/HTML/xamEditors_Masks.html

Andrea 

A MaskEditor will spell check by default? I don't see any options in your code to turn spell checking on..

I've also tried styling the TextBox (instead of the XamTextEditor) as shown below. But this still word-breaks strangely! Attached in a screenshot, notice that 'wor' of the word 'work' is shown as a spelling error!

Any ideas?

BTW I'm having to construct my styles in code rather than XAML because I'm dynamically building all this from db tables.

textboxstyle.Setters.Add(new EventSetter(TextBox.LoadedEvent, new RoutedEventHandler(SpellCheckLoaded)));

editorstyle.Resources.Add(typeof(TextBox), textboxstyle);

...

 private static void SpellCheckLoaded(object sender, RoutedEventArgs e)
  {
   ((TextBox)sender).SpellCheck.IsEnabled = true;
  }

I created a style enabling spell-checking for all TextBoxes within the scope of my grid and got spell checking to work in the text editors.

<Style TargetType="{x:Type TextBox}">
    <Setter Property="SpellCheck.IsEnabled" Value="True"/>
</Style>

Also, John, if you are using XAML anywhere near the scope of your grid, you can define your styles there (perhaps in the container for the grid).

I do need to do this programatically - I'm building by grid fields based on lots of db settings, so the styles have to be dynamicly constructed,

How can I style SpellCheck in C#?? Doing Style st = new Style(typeof(SpellCheck)) throws an exception!

Ah got it, I was assuming the dependency property had to be on the object I was styling, but I can set it on attached objects too:

textboxstyle.Setters.Add(new Setter(SpellCheck.IsEnabledProperty, true));

So I guess my spell-checking weirdness is something to do with the underlying string, not the xamdatagrid. hmmm.

Update: I've examined the string being shown. No weird characters. Also, cutting the text from the textbox, and pasting it right back in the same box, removes the red-underline! I've tried switching the grid to LazyLoad, but no difference...