I just was talking about this on another site. The promotion started on June 5th and ended September 16th.
http://www.apple.com/promo/backtoschool/faq.html
It also says "The entry iMac 17-inch 1.83GHz Intel Core Duo with Combo Drive (part number MA406LL/A) and any configure-to-order version of this product also are not eligible for this offer."
Does that mean that you have to buy it AS IS off the site? If so then that means no extra RAM or Video upgrades which is almost a must...
How many years in a row have they had this promotion? Or was this year the first time?