The deal is this: If you're already an AT&T customer, you can buy a 3G iPhone (either 8GB or 16GB at $599 and $699 respectively), without re-upping your contract. In other words, no long-term commitment required!
There are now three different prices at which consumers can get the iPhone. Current AT&T customers who are qualified for an upgrade can get it for $199/$299, just the same as new customers can. There's also an early upgrade option for $399/$499. Lastly is the new option at $599/$699.
If you buy the iPhone at the $199, $299, $399 or $499 price points, you'll need to sign a new two-year contract and you'll be required to activate the device at the point of purchase. If you pay full retail at $599 or $699, the iPhone is yours with no contract.
Worth it? That's your call (pun intended) to make.