There is ALWAYS another model "just around the corner". The iMac usually gets refreshed 1-2X/year, and the last update was a Late 2011 model. So, a Mid-2012 model isn't out of the question, but the only real question you should be asking yourself is whether or not it's worth waiting some indefinite period of time just to get a slightly better set of specs, or if you'd be better off having a computer you can actually use now.
There's also something to be said for going with a slightly older model, in that any issues that may have cropped up with the hardware have likely been identified and flagged by Apple. They might well even have set up special repair programs that will cover the failure outside of normal warranty periods.
While there are instances where it is worth the wait, like for the unibody iMacs starting with Late 2009 models, odds are any 2012 model will just be a rather small bump in specs over the current 2011 models. As it is, Apple seems to be slowing down more and more on their computer refreshes, and focusing more time and effort on their mobile devices which make up the majority of their revenues. So it's entirely possible there won't be another iMac refresh until late this year, or they might even skip 2012 completely like they've done with the Mac Pro in 2011. You just never know, so my advise would be not to put off buying something because there might be something slightly better around the corner. Maybe there is, maybe there isn't, at some point you just need to pull the trigger and let the chips fall where they may.