How do you do TDD in Xcode?
Is the built in Cocoa Unit Test Bundle not good enough? Chris Hanson did a series of posts on Unit testing in XCode, and there's the Apple documentation as well..
The popular ASIHTTPRequest library uses GHUnit.
The advantage of using Apple's stuff is that it will improve over time and will have better tool integration, but I'm not sure it's as easy to use currently.