The network needs to support it too. AT&T has started to upgrade to 7.2 but it won't be finished soon. You'll probably see next year's iPhone before you see 7.2 HSDPA go live in your area.
Even now, AT&T throttles a lot of 3G traffic down to 1.8mbps and you'll rarely get close to that with the large strain on the network now that everyone has an iPhone 3G.
So no, right now, 7.2mbps is still just a number in the US.