Are you still enjoying that unlimited data plan on AT&T? Go on and enjoy it, because it may not be attractive for much longer. Eventually, you'll probably want to upgrade to an LTE plan, and perhaps -- if you're thinking about using an iPhone for much longer -- you'll want to make a FaceTime call over cellular data. With the introduction of iOS 6, Apple has enabled FaceTime over cellular data, but it's up to the carriers to enable it. And of course, AT&T has chosen what's perhaps the most consumer-unfriendly way possible to integrate it.
The feature will be 100% free for users of AT&T's new Mobile Share data plans, which have proven to be a fairly terrible deal for all but a few folks with larger families. But for everyone else? It's not an option, period. There's no option to pay a fee to use it, nothing. You either switch your data plan to the awful new Share plans, or you stick to Wi-Fi for FaceTime.
If you're still clinging to AT&T, it's time to ask yourself: why?