Sure, AT&T might be trying to buy T-Mobile US, and sure, T-Mobile got into the microSIM game before AT&T and Apple made them famous, but it looks like now T-Mo is actually going after iPhone and iPad owners before the former, using the latter.
Already have a GSM phone or iPhone that you need to activate—or have an unlocked GSM phone you want to use on the T-Mobile network? Just get a Micro SIM card and choose a plan. Slip the ready-to-use Micro SIM Card into your phone and you’re ready to go.
And why not, there's reported over 1,000,000 unlocked iPhones already on the magenta network.
When I roam in the US, often at trade shows like CES, I often have to switch between AT&T and T-Mo, as often as I change buildings. That's not an option for US iPhone users, but if you're willing to live with EDGE speeds and pay for a second plan you could have the next best thing.
(Though yeah, the two year contract is nuts)