They won't stay that way... The AT&T phones will lock to the AT&T network as soon as they load the GSMA IMEI database with their IMEI numbers.
All of the phones sold in the US so far come with an agreement that states you will only use the phone on that carrier's network. Now that the problems with the activation database have been resolved, you can bet that AT&T will not allow over a million phones to remain unlocked.
Apple sells Sim unlocked iPhone 4's and sometime this month will be selling sim unlocked iPhone 4S's for use in the US on the AT&T network WITHOUT CONTRACT so there is no way for AT&T to sim lock them to their network. to can take them to T-Mo (although with Edge only data) or travel any where in the world with GSM service