I'd just buy a 2011 iMac and be over with it if there is nothing released with Mountain Lion. Why wait for around for a modest update if you really need a new machine. I upgraded to the 2011 for the 27" display and TB and that isn't going to change. USB 3 will be a unimportant update once Intel starts pushing Thunderbolt on the PC side and it becomes the new affordable standard.
There is always the chance that there won't be a 2012 iMac released like the Mac Pro.
I agree. I'm waiting until ML comes out hopefully next week and I'm buying a new iMac for sure. Whichever model is on sale, I'll take it. I hope they do a spec bump but if not I'm done waiting.