Robert 'Scobleizer' Scoble handily points out that NUnit 2.2 was released at the weekend, and is available for download at SourceForge. I'm currently reading (and thoroughly enjoying) Test-Driven Development in .NET, which is a really good introduction to the subject, very easy to read and generally a good primer for using TDD. In some areas it's a little shallow for my liking though - I had high hopes for the chapter on how to build a thin Web UI on top of a more complex middle-tier (too much business logic in code-behind often causes me grief), but learned nothing new. What I have recently discovered, however, is NUnitAsp - an extension to NUnit for automatically testing ASP.NET pages which I'm looking forward to giving a spin the next time I have a page to develop.
Update [2004-08-15] - there is a companion site for the TDD book including all source code here.