What displays are capable of displaying 2880 by 1800 pixels from the new MacBook Pro with Retina display?
The current Apple Thunderbolt Display is only capable of 2560 by 1440 pixels.
There are all sorts of high end specialized displays that have this resolution for the medical, government imagery, and engineering markets.
You can see any Hyper XGA display exceeds 2880 by 1800 (but are not shipping as single monitors yet) and most Quad XGA displays (shipping since 2001) exceed the "15 inch retina" in terms of sheer pixels on one display panel/monitor.
the new MBP only supports 2560x1600 max on external displays
Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, at millions of colors
http://www.apple.com/macbook-pro/specs/ (under graphics and video support)
As of yet there are no screens for the consumer that can do more than 2560 x 1600. However, if you have the money you can buy cinema 4k projectors. The problem with that is I have no idea if you can connect that to anything other than a special graphics card that typically powers them. (Maybe with an external Thunderbolt to PCIE adaptor?)