The new MacBook Pro has a resolution of 2880 by 1800 pixels. I don't know how the GPU is connected to the monitor, but a more important question for me is: if there were standalone Retina displays, what was the maximum resolution it could considering only the bandwidth of external connections, such as 20 Gbps Thunderbolt, which is the fastest that I know of.
That would depend on a number of things. One thing that must be established, though, is the fact that thunderbolt carries a displayport signal. Source.
Since the max resolution of displayport is only limited by the available bandwidth, one can only guess how high a resolution could be supported through thunderbolt.
But since most of the bandwidth for Thunderbolt is provided by PCIe 2.0 x4 (16 Gbps), it doesn't seem likely that the displayport part of it carries more than 4Gbps (same as single-link DVI).
Compression and refresh rate would be the limiting factors, though, and with sufficiently advanced compression, retina displays could easily become viable.
Another solution might be, to include a small graphics chip in the external display. That would make it possible to transfer data over the PCIe bus, making the theoretical resolution 4 times that of single-link DVI.