Interesting discussion on Peter Seibel's experiences with TDD and unit testing. I think one factor that clouds this discussion is that people tend to conflate testing in general, with the practice of test driven development. Testing your code is a good thing; running your code a couple times is testing, though it may not be as rigorous or repeatable as a suite of unit tests. Driving your design by first writing tests... well I think the jury is out on that one. It may work in some problem domains, but not all.
I would expect Flex to work as well since it's basically an actionscript GUI library written on top of Flash. There is a mxml compiler that takes your Flex gui code and converts it to a SWF. And it sounds like LLVM takes a SWF and turns it into Obj-c code so I think it would work.