It looks like they finally got around to fixing the issue which would require you to delete everything in your Termporary ASP.NET Files folder. You know the kind...
Compiler Error Message: CS0006: Metadata file
could not be found
This is good news since I always found tha highly annoying. Previously you basically had to close Visual Studio. Kill the ASP.NET worker process, and then go delete the files. More Info can be found at the link below.
Hotfix Patch for Compilation Issues
Read the complete post at http://www.dotnettipoftheday.com/blog.aspx?id=346