The Red-Hat Green-Hat Game – (nothing to do with Linux) - Joe talks about in a concrete way about pair programming. Nice way to spread system knowledge and take advantage of the need to one-up each other :-)
I think the dynamic of having other programmers trying to break your code would enforce defensive programming even more than a test-first mentality, because the offender isn't a nameless unit test, it's the guy in the next cube who drained four straight three-point bombs when you were playing HORSE last week...