Unit Testing: What should I test?

I had some discussions with my colleagues about what to test in unit tests.

I have to admit that it was always theoretical, because we didn't have any real project experience with it. But we are migrating some VB6 stuff to .NET and therefore it is an important topic for us.
But the most emotional discussion was about: Should i test internals or only the public interface?

I would like to cite Derik Whittaker final statement in his comparison, because that's exactly my point of view: "At the end of the day this comes down to a personal/philosophical decision and both are better then the alternative....NO TESTS."