There are some bugs with the current version of SIlverlight 5 RC. One of them is a rather major one and is the one you found which breaks a lot of our controls. We have this problem when setting Content of RadComboBox, RadTabControl, RadDocking etc.. We are closely working with Microsoft on fixing the issues as Silverlight 4 applications should be fully compatible with Silverlight 5. As we are getting internal builds from Microsoft, I can assure you that this problem is already fixed. But in the moment there is no way it can be workarounded.
the Telerik team
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