John Lam talks about an NUnit 2.0 gotcha he found.
If you are using the 1.1 .Net Framework, you will need to add a setting to the nunit-console.exe.config and nunit-gui.exe.config files to use the correct CLR version (v1.1.4322 for 1.1). Add this to the above files:
<startup>
<supportedRuntime version="v1.1.4322" />
</startup>
Thanks John.