The new iPhone is 3G–but AT&T’s 3G network isn’t exactly “nationwide,” so you might want to check the coverage map to make sure that there’s a 3G network in your area.
For example, Des Moines, Iowa is big enough to have an Apple store — but there’s no 3G coverage yet. (If you’re wondering what 3G is, you can click here.) Without 3G coverage the new iPhone will use the more widespread EDGE network, and you won’t see an improvement in speed.
Where Apple’s New iPhone Doesn’t Help: AT&T’s 3G Dead Zones [Silicon Alley Insider]
AT&T Coverage Map [AT&T]