I've done a lot of thinking about this and come up with three possible scenarios for how things will play out (listed below, but not in order of likelihood):
SCENARIO 1: Sept 12 rolls around and we get one of those oh-and-one-more-thing kind of moments. We are presented with two new, sleek, redesigned iMacs. There is no retina display, there is no optical drive, perhaps a non-reflective screen. (As many point out, this is unlikely given all the products that could potentially emerge in September. A 7" iPad is one of them, but I've been hearing this for years. Perhaps the 7" iPad will be bumped back to Spring 2013 when new iPads are usually presented, making room for a totally new iMac this fall.)
SCENARIO 2: The Sept 12 event happens, no iMac is mentioned. Within weeks an iMac 2012 appears with a bump in specs, Ivy Bridge, better graphics, etc. No true redesign for iMacs as mentioned above appears until Fall 2013. (I'm starting to think this is less likely. If there is a big redesign on the horizon for the iMac, it seems unlikely that they would do a minor spec bump before then. As many have mentioned here, if it was just a minor spec bump, then I think that would have happened last June. I don't think "screen lamination issues" would have stopped a simple minor spec bump.)
SCENARIO 3: iMacs aren't mentioned at the Sept 12 event and no spec bump happens through October. A complete iMac redesign is presented in the Spring 2013. (This is what I'm thinking now is most likely. But it's all marking time with speculation until we actually get some solid news. Why can't someone leave a new iMac behind at a bar, just for some real news?!?)
If you done a lot of thinking, I guess you have a good reason why you think there won't be a refresh this year? Marketing reasons are not a good reason, it would be good marketing wise to get a new model out this year. the old one sells worse, as its outdated and people are expecting a new model all ready.