I just tried to create a new Silverlight 3 application with Visual Studio 2008 and I also have no intellisense on xmlns declarations (I didn't added references to Telerik controls). I also don't have any Telerik RadControls installed, so I don't have Telerik menu item in Visual Studio.
However, I do have Silverlight 4 developer runtime, version 4.0.50401.0 (the RTW) and I suppose it is possible that it is interfering with the VS intellisense somehow.
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