Mike H. at Other World Computing (OWC):

Including the built-in Retina display, the new 2012 MacBook Pro 15″ can run four displays at their native resolution…

Moving images and media didn’t create any lag and we were able to play video on all four displays simultaneously.

The blog shows a Retina Display-equipped MacBook Pro driving two Thunderbolt-equipped iMacs being used as external displays, and an LG-branded display connected via HDMI.

That’s a LOT of screen real estate powered by a single laptop.