This picture from Other World Computing in showing the new MacBook Pro Retina connected to 2 iMacs (which are being used as cinema displays) and a third monitor via HDMI. The setup powers 4 screens with a total of 15,680,000 pixels. The writer of the post said this:
Moving images and media didn’t create any lag and we were able to play video on all four displays simultaneously.
This makes the Retina MacBook Pro the first Mac — other than a tower-based workstation like the Mac Pro — to natively power four displays simultaneously.