What if it isn't just AT&T being a nice guy? What if before the end of 2010 Apple announces support of another wireless carrier? Was AT&T trying to lock-in as many people as possible ahead of the alleged Apple announcement coming this fall?
Really at this point, what's in it for Apple to limit the iPhone to a single carrier? It's not like they're trying to preserve the good user experience.. The only possible reason is that at this point, they can't keep up with the demand for users from just a single carrier.. (unless that's a frustrating marketing ploy)