I was finally able to reproduce my problem for Apple, and I took it back in with the screen frozen. They did some quick diagnostics and still didn't find anything wrong, but the replaced the logic board anyway. I've been using it for a week and a half now without a single freeze.
I'm still not completely convinced the problem is fixed, but it certainly seems that way, and I'm getting more convinced every day. This is the longest I've ever used it with an external display and OS X 10.6.3 without a freeze. Previously the longest I went was 3 days, and most of the time it froze once a day.
(I was able to use it with 10.6.4 for an entire month without freezes, so I don't know what the deal is there. Either 10.6.4 did something to trigger it less often for me, or it was just an odd coincidence. When I was trying to reproduce the problem for Apple, I couldn't get it to happen until I downgraded to 10.6.3, so I think it really does happen more often there—at least for me.)
For those still having problems I would really recommend talking to Apple—call them or visit your Genius Bar. Even if they're not sure what to do, they might be able to fix it. It seems like most people posting in this thread had the problem fixed with a new computer or logic board replacement. Even if it does turn out to be a driver or firmware issue, the more complaints they get, the sooner it will get fixed!