I guess this is the simplest possible route short of getting a candy cell phone and pretending it’s real. AT&T is sending out just SIM cards for you to put into your already-unlocked or AT&T-restricted phones. You pay them five bucks and sign up for a contract (natch) and you get a tiny piece of plastic in the mail that tells the towers “yo, this phone’s AT&T, dig” or something similarly anachronistic (SIM cards use dated slang). Not exactly new – but now you can do it online.
AT&T now offering SIM-only option [Boy Genius Report]