To everyone in this thread,
I have been interested in ordering this phone online for a while because I don't trust AT&T handling of Android phones (and I want one now
).
I was unsure about the situation with the 3g bands, so I did some research and called At&T tech support.
After finding out that the 1900 3G bands will work by way of research, I called AT&T tech support hoping for official word on this. I was finally able to get someone to enlighten me a bit about this (one customer service gal didn't even know what 3G 'band' is).
1900 will work but the advantage of 850 is that it will work in some areas where 1900 won't work, so you won't get spotty coverage. This may not be an issue when it comes to major cities, and by looking at the coverage maps
GSM Coverage Maps, it doesn't seem like it would be a problem. However, I read on some other forum that AT&T is interested in furthering 850 and not so much on the 1900 bands anymore.
Another issue is that 850 will work much better indoors due to its low frequency.
The other issue is that even though the phone is unlocked, the settings on the phone will need to be changed in order to work with AT&T. The tech support lady searched her database for Samsung Galaxy S settings but didn't find any. She said that using the settings from Samsung's other phones will most likely work.
In other words, there is no official guarantee on anything, it most likely will work, but buy at your own risk.