This was posted on the Captivate forum and I don't know if this is true. I'm actually looking for some people to try this feature as I don't have the I9000 (yet).
When I opened a service menu (global version i9000), there was an option to enable 850MHz HSDPA band. It is disabled out of the box. I know that it works, because, at the place where I tested it, I couldn't get 3G data - only EDGE mode worked. When I enabled 850 band, phone found 3G network. Thus hardware is quad band for both GSM and 3G.
Could someone test this feature? I mean REALLY test it? Test this feature in an area where only 850 MHZ is avaiable? I think someone using the i9000 on AT&T will be able to test.