Thanks to a contribution from Microsoft, kindly arranged by Stephen Walther, I now have a copy of Visual Studio 2008 - and NUnit has a VS2008 build! It's available in CVS and will be part of the Alpha-3 release in a week or two. NUnit operates pretty much on a shoestring these days, now that I'm semi-retired, and contributions like this help a lot.
As with all of NUnit's Visual Studio builds, the VS2008 build is provided for the convenience of folks who want to build their own copy of NUnit. Our "official" releases are all built with NAnt and anyone who wants to work that way may, of course, do so themselves. Last time I checked, you could also build NUnit with one of the Express editions, so you have lots of options.
Currently, the VS2008 build uses .NET 2.0. I'm not envisioning a separate build for .NET 3.5 until NUnit 3.0. Of course, that could change if we come up with a good reason to change it.