Apple has a 14-day return policy. If they were to release a new iMac right after the back-to-school promotion ended on Sept. 21, they'd probably have many more returns to contend with than usual. So, I'd say at least two, and probably three or four weeks after the 21st, which would be the 3rd or 4th week in October.
sadly i do agree =[but then again how many people actually know that there is a new iMac? how many actually care? So maybe it won't be too bad and we could still see an earlier release.