It looks like AT&T's new SIM cards will be double-porferated, meaning you'll be able to pop it out along the MiniSIM lines to fit in current and previous generation iPhones (and most other phones), or along the smaller MicroSIM lines to fit the iPad Wi-Fi + 3G and if the leaks are correct, the 4th generation iPhone to boot.
The two SIM card sizes have always been compatible and we've heard for a while that jackets would let MicroSIM's work in MiniSIM devices and that brave souls could grab a knife and cut some plastic off MiniSIM's to make them Micro, but AT&T providing an official solution is all shades of awesome.
(Just keep in mind if you snap out the MiniSIM now, you can come back and snap out the Micro later. If you snap out the MicroSIM now, you might need some duct tape to get the rest of plastic back together later).
The new double-porferated SIMs aren't in stores yet, but should be soon.