I have been using xUnit as my default unit testing framework for some time and really enjoy its integration with ReSharper. Sadly, however, Brad Wilson seems to have to waste development cycles on re-integrating xUnit support for ReSharper everytime JetBrains comes out with a new release of ReSharper. xUnit 1.0.2 recently just supported ReSharper 4.0 and now with ReSharper 4.1 we have lost the xUnit Integration.
The Installation Utility for xUnit.net shows the sad story. Indeed ReSharper is installed, but it is ReSharper 4.1.
I wish JetBrains could help a brother out, notably Brad Wilson, such that a new maintenance release of ReSharper does not always screw up the ReSharper and xUnit love affair. It is very odd that there is such a tight coupling between ReSharper versions and xUnit, and my gut tells me that JetBrains can help alleviate this problem with a change in the way it supports 3rd party unit testing frameworks. I remember similar issues with MbUnit ( another great unit testing framework ).
In the meantime, if you use xUnit with ReSharper, you may want to wait until xUnit fully supports ReSharper 4.1 before installing it. Somebody has unofficially created a patch for xUnit to work with ReSharper 4.1 if you are interested ( xUnit Discussions ).