No sense in paying more to upgrade than it is to just change to a better carrier. T-Mobile's Bring Your Plan promotion is an amazing deal ($69.99/mo. for unlimited talk, text, and data). T-Mobile is currently refarming their spectrum to 1900 mHz so the iPhone can use its mobile broadband infrastructure meaning users won't be stuck with 2.5G Edge speeds anymore. It's worth considering for anyone who can't upgrade early due to AT&T's ridiculousness of making 4S owners wait until 2013 to get the iPhone 5, by which time the next iPhone would already be in production.
http://explore.t-mobile.com/phone-sim-card
http://explore.t-mobile.com/phone-sim-card