If you're in the States, and you want an iPhone 3G, and you're curious whether you'll be paying 50% more per month for an unavailable service, here's some help. Note that what AT&T says you're getting and what you think you're getting are not necessarily the same.
AT&T's interactive map of cellular coverage. Tick the 'Show 3G Coverage' button to see whether the big blue smear of high-speed data covers your house, workplace, and coffeeshop.
Cities Supporting AT&T 3G/Mobile Broadband. This doesn't necessarily agree with the map — for example, my city's covered on the map, but not named on the list. Click on your nearest city and see what the map indicates.
As of this posting the site's getting hammered.