Except in the case of the
iPhone, you're paying full price for it in the first place, and there isn't an "official" unlocked version available.
When I bought our Trēos last spring, I signed up for the 2 year contract and took the discount. If I wish to change providers when the contract is up (which I probably won't, I'm pretty happy with Cingular/AT&T), I'll just use one of the web services that unlocks phones for $30~40. My understanding is that law here in the U.S. (may vary by state) requires providers to unlock phones upon request once the contract expires, though they are allowed to put prerequisites on that (payments up to date, a fee, etc.). I figure it's easier and possibly cheaper to just go with the service. And I feel no qualms about that. I won't have done anything, and I'll have fulfilled the original agreement completely. They'll keep my business so long as they continue to treat me well, which is as it should be.
I'm not sure why they bother with locking them any more, frankly: you're already bound by the contract, and if you decide to ditch them at the end, you get to take your number with you, and you can get a free phone for signing up with the next guy. As more and more folks figure out they can
unlock the things, it'll get increasingly pointless. Maybe they'll give it up eventually.