The Problem was in a DropDownButton inside the RadComboBox's ControlTemplate that used to apply the ApplicationTheme's Foreground on the selected item. We have already fixed that and the properly working combo will be available in the Q1 release.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!