This news is so hot that at the time of writing, NUnit's official site isn't even updated to reflect it yet :-)
Yes, Beta 2 of NUnit 2.2 is out now on SourceForge. NUnit is the most-used unit-testing tool for .Net available today, and it supports Mono too (although Beta 2 of 2.2 doesn't). It seems Microsoft agrees it's very good, because it has hired its main author, James Newkirk, to lead Visual Studio.Net 2005 ("Whidbey") 's unit-testing feature team.
In the meantime, my currently favorite build tool NAnt is almost into version 0.85, which will support .Net 2.0!