I added a RadRichTextBoxRibbonUI component, followed by a RadRichTextBox itself from the latest (2010 Q3) build of the Silverlight controls and found that, when I had connected the 2, most but not all of the Ribbon bar buttons were working.  But why not all?

After some digging it looks like the Bindings for the Commands are incorrect and using the old deprecated Commands form the RichTextBox directly.  It seems that these have been moved to a Commands namespace and all the new Commands are only found here.

After changing the spell checking binding from {Binding Path=ToggleSpellCheckingCommand} to {Binding Path=Commands.ToggleSpellCheckingCommand} it started to worked as expected.  You may wish to update the generation of the RichText Ribbon in a future release to fix this problem.

Of course it would have been easier if I had bound to the 'Commands' path in the first place.  This is what happens when I upgrade from an older version (but I do need that Tables functionality).

Anyone who has a similar problem as I had just remember to bind the DAtaCOntext of the RibbonMenu to 'Commands', as in the below:

DataContext="{Binding Path=Commands, ElementName=editor}"