A new unit testing tool has been released to public (it's been a while actually). The new tool is called NUnitLite. It's inspired by NUnit, but it will use minimal resources and be suitable for execution on various classes of devices. Since it's based on NUnit, it will have most of the basic feature of NUnit, but please note that this package is delivered as a source code only, so you won't have GUI like in NUnit, multi-threading features, etc.
One good news is that it will support .NET Framework (including Compact Framework) and Mono. Great application for unit testing.