The Collaborative International Dictionary of English v.0.48:

Rocket scientist \Rock"et sci"en*tist\, n.
   a very intelligent person; -- used mostly in the phrases:
   "you don't have to be a rocket scientist to . . ." and "it
   doesn't take a rocket scientist to . . .", meaning that the
   subject that follows is easy to understand.
   [PJC]

WordNet (r) 3.0 (2006):

rocket scientist
    n 1: a clever thinker; "you do not have to be a rocket scientist
         to figure that out"
    2: an engineer who builds and tests rockets [syn: rocket
       engineer, rocket scientist]