Cocoa unit testing can be a pain. In addition to the usual difficulties of writing tests for user interface heavy code, the Apple sanctioned solution, SenTestingKit, can isn't the greatest testing framework around and the default way of using it rules out debugger.
GHUnit helps somewhat. It's a GUI test runner with additional testing methods. However, with it you are still writing your tests in Objective-C. On the plus side it's the same language you're probably writing your app in. On the minus side Objective-C can be verbose and sometimes, especially when writing test code, brevity would be welcome.
Enter MacRuby. You get the conciseness of Ruby with full access to your Objective-C classes. And Ruby probably has the greatest density of testing frameworks per active programmer among all the languages in popular use today.
There's a nice article about TDD, Objective-C and MacRuby on the MacRuby site. However, the approach taken in it still uses a Xcode build phase script to accomplish testing. That makes debugging hard and you have to hunt through the build logs for your errors.
RCRunner is a separate GUI program you run. You tell it names of Ruby modules and it uses any test cases it finds. You can breakpoint your code and thanks to Ruby, reload the test code. You can inspect errors and log output test by test.
 At the moment it supports only MiniTest but adding support for other frameworks isn't difficult.