Even an unlocked 3G iPhone will still be an AT&T iPhone

AT&T uses the 850/1900 MHz frequency spectrum for 3G data (850 MHz for voice). The only other major GSM carrier in the U.S., T-Mobile, hasn't yet rolled out its 3G network, but when it does (rumors indicated the switch may be flipped tomorrow) it will use the 1700 MHz spectrum for download and 2100 MHz for upload (850/1900 for voice) and thus no AT&T capable 3G devices will give you full 3G data access with T-Mobile. You may get EDGE (2.5G) data access and you will be able to make and receive calls though.

Also, Europe and other countries use a 2100 MHz frequency for high speed data, but even then this is different than the U.S. T-Mobile spectrum (now that can be confusing). Then we have Sprint, Alltel, and Verizon who are all CDMA carriers that have a completely different infrastructure and won't be compatible with the 3G iPhone.

There is a slight possibility the 3G iPhone will have a tri-band 3G 2100 MHz radio and if that is the case then you could use 3G data in Europe and when you travel. It still will not work with T-Mobile's network though and thus will have limited carrier support in the U.S. whether it is locked or unlocked. I suppose Apple could also add in another radio (and antenna) for T-Mobile USA, but since this is the smallest U.S. network and has its own band I don't think I would get my hopes up for this scenario.

The next version of the iPhone will be a 3G iPhone and AT&T may raise the monthly data fee, however with Sprint and Verizon already offering 3G data at US$15 and US$30/month I don't think AT&T can raise it too much more and remain competitive. Would they then have a standard iPhone data rate and a 3G iPhone data rate? We'll just have to wait and see. I am sure we will continue to see lots of rumors as we wait over the next month or so for its release.