Last year, the back-to-school promo specified particular part numbers for eligible macs, so that when the iMac was upgraded in September, the new model was not on the specified part number list and was not eligible for the promotion.
This year, however, there don't seem to be any part numbers for the Macs specified in the promotion; only for the iPods. Therefore I hopefully conclude that the iMac will be upgraded soon (WWDC Please-oh-please) and will remain eligible. That would make me a Very Happy Camper.