Generally, failed validation does not depend on the type of element in the CellEditTemplate. In this sense there should be no difference whether you are using AutoCompleBox or RadComboBox. Are you capable of reproducing your behavior on the sample attached previously ?
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!