The Other World Computing blog ran an experiment with the new Retina Macbook Pro:

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

1. Retina on laptop @ “best for Retina”
2. iMac used as a display @ 2560 x 1440 via Thunderbolt
3. iMac used as a display @ 2560 x 1440 via Thunderbolt/DisplayPort
4. LG monitor @ 1920 x 1200  via HDMI

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

Impressive.