Sadly, I think retina iMacs are further away than we'd like to hope. Eizo has a 36 inch 4K monitor for $35,000¹. There's a 31 inch 4K Viewsonic Monitor for an undisclosed price². Even that's only 150dpi. If Apple wants to make a pixel doubled 27 inch iMac for under 10K, I'd be surprised it if they could do it by 2013.
But then again, I thought the retina MacBook Pro was impossible. So perhaps I'm not the best judge of these things.