Google's HTC Nexus One, while available unlocked (always), but subsidized by T-Mobile with a two-year contract. Hopefully anyone who bought the Nexus One with the intent of using it with an AT&T SIM read the fine print and realized that it wasn't compatible with 3G on AT&T's network. That may be changing, though.
Versions of the HTC Nexus One for Verizon and Vodafone have been listed as coming on Google's website, but if you're going to have the darn thing unlocked and its a GSM device, it's a pain that AT&T users can't have 3G support on it. However, a recent FCC approval
seems to point to an AT&T 3G compatible version of the Nexus One.
The problem, of course, is that to get 3G working on both T-Mobile and AT&T means coverage of more frequency bands. This could compromise size and weight of the device which could be the reason Google went the way they did.
At any rate, the Nexus One has not been a blockbuster, at least so far, which is probably a good thing for AT&T users: they don't need extra app phone traffic sucking away all their bandwidth from their iPhones.