I hate to tell you, but you can roam on the Cingular network with ATT. I have ATT and my Audiovox 5600 basically switches between the stronger signal, Cingular 850 or ATT 1900. This is seemless to the end user and part of the benefit of Cingular/ATT merging. It is a selling point and NOT considered roaming.
I would be willing to trade you your phone for my Audiovox 5600. I am moving to Europe in 4.5 months and need the two European bands, while you need an American phone with the two american bands. The offer is open to anyone, my Audiovox 5600 is unlocked and ready to go for a European HTC.