iPhone 3GS buyers by just a little bit. If you purchased a 3GS -- either the 16GB or 32GB model -- between May 7 and May 14, you'll have until June 14 to get credited the difference between the price you paid and the $149 / $199 they're charging now; if you bought it after the 14th of May, you'd have at least until the 14th of this month anyway, so it's effectively a one-week extension. If you'd rather trade up to an iPhone 4 rather than pocketing some cash and keeping the 3GS, you've got an option there, too: anyone who picked up a 3GS between May 7 and May 21 can preorder an iPhone 4 by June 21 and get the new phone for only the difference in cost, assuming you bring in your 3GS when you pick it up.
On a related note, modmyi.com is also reporting that AT&T corporate stores are set to open at 7AM on the 24th, iPhone 4 launch day, though we're not able to confirm this one. Word on the street is that new customers will only be able to activate one line and existing customers will be able to buy one iPhone 4 per line of service, clearly an attempt to stretch inventory as far as it'll go. With the way Apple's setting up for preorders and launch-day delivery, it seems like a lot of this chaos and bedlam is going to be avoided -- but hey, it's Apple. You never know.