9to5 Mac recently discovered that Apple is no longer offering no commitment pricing for the iPhone 4 online. This means that you can no longer grab an iPhone 4 for $599 (16GB) or $699 (32GB) without it being tied to AT&T. The blog believes that this could be Apple's response to the recent Jailbreak, which allowed anyone to jailbreak their device without syncing it to a computer.
This information was discovered on the iPhone FAQ section of the iPhone 4 online store page. The page mentions that you can't buy a no commitment device because the "iPhone requires a two-year AT&T wireless service contract."
[Update] Apple has contacted 9to5 Mac to say that you can actually purchase the no commitment iPhone 4 in the Apple Store, just not online.