The RadControls for Silverlight installer does not integrate the Toolbox items to the Dev Preview of VS11 and the VSExtensions for RadControls for Silverlight don't plug in the new IDE, but in terms of controls usage there are no limitations as far as you target Silverlight4.
Still, VS11 is in a Dev Preview stage and I wouldn't recommend using it as your main development platform.
the Telerik team
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