Currently, the GridViewComboBoxColumn cannot pull data correctly from a DataTable. I have logged this issue and our developers will try to implement this feature in time for tomorrow's internal build. Until this feature is available, the only workaround is to use a collection of business objects as the combo's ItemsSource.
I have just talked to our developers and they assured me that the DataTable issue will be resolved for the upcoming 2009 Q2 release which is only a few weeks away. Please, excuse us for the inconvenience.