If you tried to install the Visual Studio 2010 Express project and item templates of MVVM Light V3 SP1 (with the Windows Phone tools hotfix), you may have encountered an error when unzipping the package, telling you that a file was corrupted. This error was reported to me a couple of times, and I was able to reproduce the issue.
I just published a new version that takes care of this issue. If you encountered that error when installing, please download the new version and try again.
Note that the error was only there for the Visual Studio 2010 Express version of the templates. No other changes were made.