I'm not sure if this a 64 bit problem, a Windows 7 problem or just a VS2008 SP1 problem but I have been pulling out the little hair that I have left trying to get the intellisense working for XAML files on my new install. At best, I could get the files to be opened up in the XML editor, but the WPF editor was missing.
Fortunately I found this post. There were plenty of partial solutions but strangely two solutions had to be combined
1. Run devenv /setup from the VS command prompt in admin mode
2. Follow this with devenv /resetskippkgs