Wow! I heard a rumor that this was coming, and I must admit I thought it was hogwash.
I can't understand why Apple would do this. The only new carrier it adds for the iPhone 4 is T-Mobile, which 1) won't support 3G with it, 2) might become a division of AT&T in the near future anyway, and 3) probably didn't specifically authorize this move and could potentially react negatively to it.
I can't help but think that the average consumer will see this as a much more expensive option with no real advantages, although it is sort of cool to think that the option is there.
Now, not requiring a contract is a real advantage, but AT&T (theoretically) already offered a no-commitment price option on the locked model, so that's not really a new advantage for this product.