Gavin, have you abandoned any effort to utilize Stefan's bitcoinjs/nodejs stuff for this testing framework?
I never started with bitcoinjs/nodejs, because I'm not a JavaScript programmer (and I'm too busy to take the time to learn).
It would be lovely if somebody who is a JavaScript programmer decided to show me how easy it would be and took bitcoinjs/nodejs and created a really nice cross-implementation testing framework with a bunch of test chains.
RE: Mike's point that creating full-difficulty test chains is a pain in the ass: I think that one-time pain is worthwhile, because I don't think we can count on every bitcoin implementor making it easy to run their implementation in a unit-test mode with super-low difficulty. And creating difficulty-1 blocks with a GPU is actually pretty darn quick...