I ran our app tonight on a new (to me) laptop and was greeted with a nice error page saying "Mutex Could Not Be Created". I Googled the error and found the following solution:
http://blogs.msdn.com/jorman/archive/2006/07/24/system-invalidoperationexception-mutex-could-not-be-created.aspx
The problem here is that the registry key that contains the compiler mutex (not exactly sure what that is, anybody care to educate me?) hasn't granted the necessary permissions to the ASPNET Worker Process (or whatever account under which your app pool is running). The fix involves running ASPNET_RegIss -a <username>.
I tried it out; no such luck :(. Looked through a few more of the search results and found a completely different solution to the same problem:
http://windev.wordpress.com/2007/02/
Basically, this problem consists of Visual Studio locking some temporary files stored in %WinDir%\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\<AppName>. Closing Studio, deleting the temporary files, and restarting IIS is the fix.
I tried this one; BINGO! It fixed the problem. So if you're stuck on this error and the first solution doesn't work, the second might.
Best of luck!