I ran into an issue the other day where I was trying to step into a method while debugging a unit test and the debugger just stepped over the method call.
The method was in a class library that the test was exercising. The problem was fixed by disabling Code Coverage. What I did was went Test->Test Settings-> and unticked the test settings that were current (that had code coverage enabled).
This has also been noted on Microsoft Connect so hopefully we will get some kind of patch soon.