Michel,
I'm not sure why the dual 2.5GHz systems w/o the 6800 are being delayed so long.
In my case, I placed an online CTO order for a dual 2.5GHz PM with the 6800 and 30" monitor on 8/18. On 9/21, I received an email indicating a delay on shipping to 10/22 (which is the same delivery date as the 30" display). While not actually saying it, the email strongly implied that the delay was due to availability of the 6800 card.
I called the Apple Store (on 9/21) and after some discussion convinced them to breakout the 6800 card. After they changed the order the ship date for the G5 system (w/o the 6800 card, but with the low end ATI card installed) was set to 10/5 and the 6800 card, now separate, is 10/22 (the same as the 30" monitor).
I noted on the online order status that my G5 system shipped on 10/4, a day earlier than the expected date. FedEx tracking shows my system on a truck for delivery today. (This is FedEx Ground, but since the system shipped from Sacramento, CA, the transit time is only a day or two from there to my home).
So it seems that delays could be due to the 6800 cards. But then, who knows for sure.
Jerry