The price dropped on the iPhone this morning to $399 (link to Engadget). I am sure ATT knew about this before hand.
I am betting that it will take ATT 24+ hours to update their website. It was announced at 11:10am Pacific Coast Time. This time tomorrow the page will probably be the same. I can just imagine customers going to the site ready to buy the phone and it still shows the old higher price for the 8 gig model. Great customer service ATT, keep it up!
++Update: After checking their site about every hour the prices were updated at 6:30 Pacific Coast Time.++