Plus, I would like to point out that Of all the iPhones I have seen on Ebay, None of them sold for less than normal retail. Most were higher.
Personally, I would recommend buying from Apple or AT&T and getting the 2 year standard warranty, because you probably will not have one with something sold off Ebay.
There is a post on here somewhere where some guys bought an iPhone off the internet on Ebay, and AT&T ended up biling him for the phone again because of some "jacked-up" misunderstanding. I would want to avoid that scenario like the plague.