Thank you for the feedback.
Indeed, the problem with Blend toolbox items can be reproduced with the latest official binaries. We will resolve it immediately and the fix will be part of the next internal build (on Monday).
As a temporary workaround you can remove Telerik.Windows.Controls.GridView.Design.dll located here: ...\Telerik\RadControls for Silverlight Q1 2012\Binaries\Silverlight\design and all of the GridView elements will be shown correctly.
Please, excuse us for the inconvenience caused. I've updated your Telerik points for the valuable feedback.
the Telerik team
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