Apple does not wait for OS releases to release new hardware, even a few months. This happened in 2007 with the aluminum iMacs (August release) and Leopard (October release). They will just get a special build of Snow Leopard if there is unique hardware. Thunderbolt, Sandy Bridge and probably updated graphics are definitely worth waiting a few weeks, IMO. This could be as big a change as going from Yonah to Merom in late 2006.
Did they give some sort of discount on Leopard to those that purchased the iMac in August? Is it pro-rated? For example, if a normal upgrade is $130 and it is free for those that purchased an iMac under 30 days, would be it $65 for those that purchased an iMac 2 months before an OS release?