I noticed the other day that that XAML Intellisense had stopped working in Visual Studio 2008. A quick web search revealed that others had experienced this problem after installing a version of the Windows SDK. I installed the Windows SDK for Windows Server 2008 a couple of weeks ago so it may have been the culprit. A bit more digging revealed that the Windows SDK team have published a workaround for this problem over on their blog. I followed the workaround and XAML Intellisense is now working again.