Using Visual Studio 2008, IE9 and Test Studio 2011.2.1413 I was not able to reproduce your issue. Please check the following article and make sure you follow the steps:
You can also upgrade to latest version of Silverlight
If you are facing the problem again please provide us with access to the application in question and step by step instructions so we can try to recreate the problem.
Looking forward to hear from you.
All the best,
the Telerik team