I interpret this as an indication that the new iPhone won't be available immediately. Since AT&T has dropped the 3GS to $99 but there is no info on the newer iPhone (AT&T could have left the website the same way it was yesterday, but instead they changed it), I interpret this as AT&T's efforts to clear out 3GS inventories before the new iPhone comes out. Why would someone by the 3GS if there's a new iPhone? Until a few days ago, the original 3G was still being sold - and, allegedly, the 3GS will continue production.
Just some thoughts. We'll know for sure tomorrow.