I have been working recently to introduce JUnit and NUnit to my development shop. My findings are pretty interesting. My Java developers jumped all over JUnit, and my .Net developers were substantially less enthusiastic about NUnit.
I've also done some work with NUnit myself, and have found that it's a pretty interesting platform -- in my case not for unit testing as much as for integration and acceptance testing. In fact, I've wondered if "JINtegration" and "NIntegration" can be far behind.