Just wanted to point folks to a fix I just found helpful:
Thanks to http://blog.tfanshteyn.com/2008/01/fixing-wcfwpf-vs-2005-extensions.html
Basically, once you've installed VS2008 or .NET 3.0 SP1, the registry key saying that .NET 3.0 has been installed is removed. If you want to, say, install Visual Studio 2005 extensions for .NET Framework 3.0 (WCF & WPF), November 2006 CTP, you'd recevie errors to the effect that .NET 3.0 wasn't installed, and was a prerequisite.
Thanks!