After a couple of statuses (stati?) about how pleased I am with my (current) iPhone, I've gotten several questions about buying an unlocked phone to use overseas. As was my purpose with this blog in the first place, I'm going to park my answer here to link to in the future.
I am currently the (proud and not-so-proud) owner of two used, unlocked iPhones. So, I can tell you how to and how not to buy one.
How not to: look randomly for a used, "unlocked" phone on eBay, buy it, bring it to the country where you live and have a partially unlocked, partially functional phone/iPhone (mine never recognized wireless, which negates almost half the functionality of the iPhone and didn't wake up from sleep to ring, so you constantly had to check to see if you'd missed calls, if you wanted to put it to sleep to, say, put it in your pocket).
How to: Buy used from Gazelle on eBay. You can check them out at gazelle.com. They buy used electronics from people, evaluate them, and turn around and sell them (they also do fundraisers where non-profits can collect used electronics from people and get paid for them, which I thought was cool). The advantage they have over buying used on eBay, in general, is that they have a consistent rating system, they deal in bulk, so they know how to evaluate the condition of the piece of equipment, and they detail the condition of the piece, i.e. where scratches are, etc. I like this, because you can decide that you don't mind scratches or an engraving on the back, for example, since they'd be covered by a case, but you don't want scratches on the screen. We've bought three 3rd gen iPods and two iPhones from them, so far, and been very pleased.