We are aware of this issue and there is already a fix available for it in Q1 2012 SP1,
you can download it or 2012 Q2 Beta
version of RadControls and see how it works on your side.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed