With the original 0.86 beta 1 release, there was a small problem with folks who only had VS 2008 installed because there was a dependency on a registry key that was installed with the .Net 2.0 SDK (or VS 2005).
I have to say that over the past 2.5 years, I have been very pleased with NAnt. It is a great open-source build tool, and it integrates with every other command-line tool I could want. I normally <exec/> out to msbuild.exe for the compile. I have a NAnt build template available in my public source code repository here.