Dr Sylvester James Gates, Professor of Physics at the University of Maryland. He has a BS in physics and mathematics from MIT as well as a phd in physics, also from MIT. I first found out who he was while watching a three part NOVA special on string theory (yes, ladies and gentlemen, in case the title of the blog didn't give it away, I am a geek)
Apparently, he's one of the foremost experts in the field of string theory. He's also considered one of the best there is at explaining the concepts behind string theory to the general public.
Now, for those of you who are less technically inclined, string theory is an attempt to explain all structure and interaction in the universe in terms of vibrating 'strings'. Its an extremely mathematically intensive field and as close as you can get to cutting edge science in theoretical physics these days. The people involved in it spend most of their time dealing with concepts that can be fully understood by maybe 20 people on the planet.
He's actually pretty high up on the list of people I want to meet. The next time I'm in Maryland, I'll probably swing by the college and sit in on one of his classes.