I installed all the necessary files for Silverlight today. When I created a project, the xaml document didn't have intellisense and opened as a text instead of XML-based document.
I tried a few fixes, one guy recommended right-clicking on the file and choosing "Open with.." and selected XML Editor as the default. This didn't work for me.
I tried following Karen Meier's fix here. That didn't work either.
Then, I figured I would try resetting Visual Studio. So, checking the documentation on devenv switches, specifically this one, I ran devenv /resetskippkgs and checked the intellisense. This fixed my XAML intellisense!