Good answers all. I would add a few ways to avoid the dark side of TDD:
I've written apps to do their own randomized self-test. The problem with writing specific tests is even if you write lots of them they only cover the cases you think of. Random-test generators find problems you didn't think of.
The whole concept of lots of unit tests implies that you have components that can get into invalid states, like complex data structures. If you stay away from complex data structures there's a lot less to test.
To the extent your application allows it, be shy of design that relies on the proper ordering of notifications, events and side-effects. Those can easily get dropped or scrambled so they need a lot of testing.
