IMO they will ship the new iMac in September, I hope the screen is matte or reduced glare as I work with a window right behind me and I can't change that.
I agree the retina display is unlikely this year unless they have a massive increase in the speed from the GPU. I'm betting the retina iMac in an all new case will come mid 2013, likely without DVD but will probably keep all it's current ports on the back. The main reason some were dropped from the MBPr was a lack of space, not an issue on the iMac. It would be nice if they placed 1 or 2 USB ports on the side near the SD slot for easy access. I also bet the 2013 iMac will feature a thinner case, which could fit a DVD drive but Apple is keen on people getting there music/movies from iTunes. Plus with a thinner case cooling could be an issue so no DVD could allow better air flow. Also I doubt they will solder in the RAM, again the main reason they did this in the MBPr was space issues.
So IMO the 2013 iMac will have.
- New thinner case, reduced or no 'chin'.
- No DVD.
- Retina display.
- User upgradeable RAM, 4 slots.
- Existing ports on the back.
- SSD cheaper, maybe standard on all but the base model.
- Faster CPU/GPU/RAM.