If you add the .NET framework 3.5
compatible assemblies (from Bin35
folder) the controls won't be visible in the toolbox of a website whose target framework is .NET 2.0
and vice versa.
Please examine the following links for more information:
Adding RadControls to the Visual Studio 2008 Toolbox
Using RadControls for .NET 2.0, 3.0 and 3.5 projects in Visual Studio 2008
the Telerik team