Even if AT&T doesn't make you sign a two year agreement with a used phone I'm not sure that is such an advantage. It costs $175 to end your contract early if you need to($175 at most, it goes down $5 for every month you're in contract I believe). Plus, why is it so bad to have a contract anyway? You need *someone* to provide service and I don't think anyone else's plans are any cheaper.
The only other network the 3G could theoretically work on is T-mobile and there isn't an easy unlock for the 3G right now--though I understand its coming.
I'd just get a new iphone--unless someone is selling you the 3G at the subsidized price *they* paid for it--or less. I don't see any advantage.
The advantage is knowing that there will be absolutely *zero* penalty to get out of the service early if your circumstances ever change to the point where you can no longer afford to maintain service.
$175 penalty, or $0 penalty. Pretty clear advantage, if you honestly think there's a strong probability of a change in financial circumstances.
As well, last time I researched the topic, there were no technological measures in place which would prevent the iPhone from working with an AT&T SIM card registered to a prepaid account. If one of prepaid pricing options meets your fancy, that might be an attractive alternative.
Of course in that configuration, after the MEdia Net Unlimited offer expires, AT&T will not carry any unlimited data plans which could be used in conjunction with its prepaid service. So unless you shut off data entirely (either using WIX BLOCK or by configuring phony APN settings) it might get pretty expensive.
(Unlike AT&T's postpaid terms of service, which has a section dealing specifically with mandatory data plans on the iPhone, AT&T's GoPhone terms of service doesn't have anything to say about mandatory data plans, no matter what brand of phone you're using.)