Xcode - Debugging a unit test target
Using XCode 4.2, (with SenTestKit unit tests as set up by checking the "Include Unit Tests" checkbox when setting up the project), Product->Test wasn't hitting my breakpoints, but Product->Perform Action->Test Without Building seems to do the trick. Hope this helps.
The link posted by David Gelhar is correct for Xcode 3.
For Xcode 4, things are much simpler. Edit your current scheme and go to the "Test" action. Click the '+' at the bottom and add the test bundle that contains the tests you want to run. Now when you choose Product -> Test, it will run those tests. Any active breakpoints will be hit just like you'd expect.
Here's a blog post: Debugging Cocoa application unit tests with instructions for how to do this (for XCode 3 at least; not sure about XCode 4).