After I installed Visual Studio 2008 RTM in Vista, I could not use the Toolbox to add any controls to WPF forms. It's possible that this has something to do with my having previously installed beta versions but here's what eventually worked for me.
Right-click on a Toolbox panel and select Choose Items. On the WPF Components tab, you'll see that all of the checked controls are listed twice. Uncheck the checked version and check the other one. When you're done, click OK. Now the Toolbox should work.
Strangely enough, if you again right-click, select Choose Items, and go to the WPF tab, you'll only see one copy of the checked controls.