If you're the type to upgrade your iMac every two years or so, then your starting point, whether it be a 2011 or 2012 iMac, in grand scheme of things wont make that much of a difference, as generation to generation only offers very incremental changes. Unless USB 3.0 is a must, or you crave retina or a larger screen immediately then stay the course. Ask yourself if you're willing to fork over another $400-$600 on top of what you're paying right now?
The rumors floating around, the specs you may desire, may very well not be on the 2012 iMac. With a 2-3 yr upgrade cycle, you're more to be "WOW"ed by your upgrade and get better value for your money.
That's how I would justify my purchases/upgrades personally.