But if I buy a new one from Apple/AT&T, in addition to the cost of the phone, I'm also on the hook for something like three years of monthly service at something like $80 a month. And it's not as if Apple's going to be receptive to fixing it under warranty once you've hacked it, which is what they consider unlocking it to be.

Of course, if you can get it on a PAYG plan in the UK, that would make more sense. However, Apple refuses to service iPhones in the US that were purchased in other countries, so that's not a solution for people in the US. Of course, if you're hacking it anyway....