NUnit 2.2 was just released. You can download it here until they get it loaded on the website.
If you check out the NUnit documentation, it explains how you can run NUnit 2.0 and NUnit 2.1 tests using the new 2.2 framework without recompiling the tests. I recommend trying that first before you upgrade to NUnit 2.2 if you have any concerns.
More Information and Resources on NUnit and Unit Testing for .NET.