Got the rMBP 3 days ago (2.7GHz 15" model with 16GB memory). Within a half hour of setting it up, the fans howled like a banshee and then subsided in a few minutes, and it's been doing this once a day. The laptop isn't any hotter than my 2011 MBP when the fans kick up, so I'm guessing something strange is going on.
I thought it might have been an airflow issue, so I tested on different surfaces (on a standard wooden desk, on dining room table with tablecloth, on bed) and after some use the fans pick up again.
The only thing I can think of (that I am doing differently) is running at the highest resolution (the 1920x1200-like resolution).
MacBook Pro with Retina display, OS X Mountain Lion (10.8.2)