I can't wait for a Retina-class 27" display. I hope this is clear to everybody who knows me — high-resolution displays were always one of those "amazing things that we'll have in the future" things. And now that I have a few, not only do I get to "live in the future" on a regular basis, but I've become impatient, and what high-resolution displays everywhere.
To that end, Marco Arment had a great blog post the other day, entitled: "27” Retina math", that I would like to chime in on.
The idea of a Retina 27" display just being 4K is appealing — presumably that's going to be the next standard for TVs, so panels of that resolution will be plentiful, as will content (eventually). And while I agree that current GPUs are barely up to the task, and Thunderbolt doesn't (currently) have enough bandwidth, I think there are two solutions that Apple could deploy, to get a large Retina panel next year (instead of having to wait until 2014, as Marco suggests):
Bring SLI graphics to Mac OS X. SLI is NVIDIA's term for using two GPUs in tandem to render an on-screen image. Basically, each GPU handles half of the screen.
Switch back to DisplayPort. The latest DisplayPort spec can bring 17.28Gbit/s of bandwidth to bear, which is more than enough to drive a 4K display. However, when riding over Thunderbolt, DisplayPort is capped at Thunderbolt's peak bandwidth limit, which is 10Gbit/s.
While the thought of dual GPUs sounds complicated, that isn't the worst obstacle. Apple rarely goes backwards — after having pushed Thunderbolt, they are probably loathe to go back to putting DisplayPort-only ports on their machines.
However, there is a way out: consider if Apple produced a 27" Retina iMac. All of the graphics chips and cabling are hidden inside of the machine. Thus, Apple could have a DisplayPort only connection from the graphics cards to the display, running at full bandwidth, without compromising the Thunderbolt ports that are accessible externally. Thus, I think that the iMac is how Apple first brings a Retina-quality display to a desktop computer, and we have to wait for the next version of Thunderbolt before there is an external, stand-alone Retina display.