The quad iMacs started shipping a couple of days ago, but they were all Core i5 models. The Core i7 iMacs started shipping today, direct from Shanghai.
I ordered later in the day on Oct. 20 and my Core i7 has not shipped yet, but my order status has now changed to "Prepared for Shipment".
Just after midnight, I got the shipment notification email from Apple. That would be early Thursday afternoon in China.