Ok, this is interesting.
Jim Tanous over at the mighty Mac Observer has just penned a how-to guide as to how to see all possible resolutions for your Retina Display MacBook Pro on an external monitor with a little tinkering and the option key.
Head over here to take a gander and as nifty as the Retina Display MacBook Pro is, you should be able to see all possible resolutions sans said external monitor.