This issue is related to a bug in Visual Studio which will be fixed soon in the 2012 version of the IDE.
We have found a workaround for it for the 2012 version and the controls that target the Windows Phone 8 SDK are now working as expected.
This issue normally won't happen with Windows Phone 7 controls and Visual Studio 2010 on our side. Are you now using Visual Studio 2012 and a Windows Phone 7 project in the moment when this happens?
the Telerik team
Have a suggestion or face a problem - you can use the Ideas & Feedback portal
to submit ideas, feedback and vote for them.