Yes but there are 2 different version of htc hd2. 1. htc hd2 t8585 and 2.htc hd2 t9193. The first version is the Europe version which support 900/2100, so that wont work for att's 3g. And the second version is the Australian version on telstra network which uses 850/2100. So if you want att's 3g network to work on a unlocked htc hd2, get the australian version which is the htc hd2 t9193. Cheers.
PS: Look for a 850MHz map coverage as it isnt widespread