KB957541 is my favorite hotfix
It is not public yet (but you can call and ask for it), but it will be when SP1 goes to Windows Update. This is the fix for the ExecutionEngineException that appeared in .Net 3.5 SP1, and was found by Rhino Mocks.
It took a while (but not unreasonably so), and it is here, yeah!
This is fixed, you can get the fix here: http://support.microsoft.com/?id=957541