Hi
Are you running on 64 bit OS?
If you are running inside VS2010 you are using the Resharper 5 beta.
This version has some issues with its test runner especially in 64 bit.
Note that MSTest runs only in 32 bit while NUnit can run both 32 and 64 bit.
Please let me know the exact version of the R# nightly build you are using and I'll check it out.