Walking around Mix08 today I had a chance to help a few people with some install issues.  In some cases the installer was asking for VS 2008 media and in other cases it was refusing to install. 

Luckily, Bradley Bartz from the Visual Web Developer team was around to help folks.  He drilled into the cause of the issues he saw and wrote up a very nice blog post listing the common issues and solutions.

Installation Tips for Silverlight Tools Beta 1 for Visual Studio 2008