Unfortunately this is a known problem in RadComboBox for Silverlight Q2 2010 SP1. We fixed it with the next internal build, so if you update the assemblies again, the problem should disappear.
I apologize for the caused inconvenience.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items