The Rad Controls Q1 2007 will be part of the official release of Sitefintiy 3.0, not the RC2 edition. RadTextBox will be included then too.
We have already thought about separating the RadControls dlls and are currently working on this.
Meanwhile, you can use Binding Redirection to avoid the exception. Here is a link you might find useful:http://msdn2.microsoft.com/en-us/library/2fc472t2(vs.80).aspx
the Telerik team