I've been playing around a bit with WPF lately (with Visual Studio 2008) and was disappointed that there wasn't any intellisense in the designer (like what you'd see in the ASP.NET designer). I happened to mention this on the UT and John Fenton there pointed me to this link:

http://blogs.msdn.com/windowssdk/archive/2008/02/22/workaround-installing-win-sdk-after-vs2008-breaks-xaml-intellisense.aspx

Apparently installing the Windows SDK after installing VS2008 breaks XAML intellisense. The link above walks through fixing it (very simple registry entry fix).

Links

http://www.universalthread.com
http://blogs.msdn.com/windowssdk/archive/2008/02/22/workaround-installing-win-sdk-after-vs2008-breaks-xaml-intellisense.aspx