A small amount of interest in your bank account. Against it: You won't have an amazing Mac until there is an upgrade. And in a few years, you will either be later getting the next much improved iMac, or you will be paying considerably more money per month of use.
All that waiting seemed kind of fruitless. So I just ordered one. I pimped it up with 8 gigs of RAM and two gigs on the graphics card. This machine should serve me well as a dev machine for at least a few years.