If you a rearing to get a HTC HD2 on AT&T the only option right now is buying one from the other side of the world, Telstra in Australia specifically.
The Australian HTC HD2 has just launched 2 days ago, but has already found its way to USA, where an excited owner has posted about his experience on the AT&T network. The phone does not work very well with the native Telstra ROM, but a custom ROM like Duttyâ€™s is said to sort out most issues.
One note one should bear in mind before considering the purchase is that Telstra only shares the 850 Mhz 3G band with AT&T, meaning the phone will not be able to access 3G on the 1900 AT&T band. Fortunately AT&T is at present expanding its 850 Mhz network as much as possibly, so this may not be much of an issue.
For more detail read this XDA-Dev thread here.