So far, AT&T has always offered discounted pricing to existing iPhone owners wishing to upgrade to the newest iPhone. Apple has debuted a new iPhone each year the last three years, and existing customers have been able to enjoy at least a small discount rather than paying full price.
The iPhone 3GS 32GB has a full retail price of $699. I paid $499 for mine. Other AT&T customers qualified for even deeper discounts on the iPhone 3GS last year, and only had to pay $299.
Apple is widely expected to announce a new iPhone later today. AT&T has obviously changed the upgrade eligibility date for existing iPhone owners so nothing will stand in the way of them purchasing Apple's new hardware as soon as it becomes available. No one has any idea how much it will cost. I wouldn't be surprised if pricing is consistent with the existing line-up of iPhones, which fall at $199 for 16GB iPhone 3GS and $299 for the 32GB model.
The new model is likely to have major improvements, including a higher-resolution display, better camera, secondary camera, better battery life and more storage.