Here's the fix to a Microsoft Acknowledged problem with ASP.NET and the Index Service.
Not too bad of a problem, but surely wiley in nature ... I killed a good few hours on this. It would only happen at random, I'd recompile, and it would give me access denied errors on random .dll's in my bin directory. The only way to fix it was to reboot, and even that didn't always work.
Good thing for Google.