What is the maximum resolution for a second monitor attached to a MBP Retina? I'm curious if my new machine will support (for example) 3840x2400 retina-quality external monitor in additional to the built-in screen or whether that will exceed the capacity of the video card.
From the Tech Specs page over at Apple:

13":


  Dual display and video mirroring: 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


15":


  Dual display and video mirroring: 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