Other World Computing posted this picture on their blog showing a MacBook Pro with Retina Display hooked up to a pair of iMacs serving as Thunderbolt displays and a third monitor via HDMI. This setup powers four screens with a total of 15,680,000 pixels.
The writer of the post, OWC Mike, seemed impressed with the performance of the MacBook Pro:
Moving images and media didn’t create any lag and we were able to play video on all four displays simultaneously.
Apple officially supports hooking three monitors into the Retina MacBook Pro, noting in the Thunderbolt ports FAQ in its Support Knowledge Base: MacBook Pro (Retina, Mid 2012) can support an HDMI-compatible device on its HDMI port while also using two Thunderbolt displays.
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. 
