RadCompression has always been a part of RadControls for AJAX and was never intended to be shipped as a part of RadControls for Silverlight. The sole purpose on our website is to show how different modules and components across our products can be used for achieving better performance. In your particular case there is not a problem to use only RadCompression from the AJAX controls - it does not have a visual part so no trial messages will appear. Of course if you need other components from the AJAX suite you'll need to obtain a separate license.
the Telerik team
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