I have already seen two very famous geeks here at DARPATech.
I would not have recognized Vinton Cerf to see him. But I happened to glance at his name tag, almost by accident, and I certainly recognized it:
Chief Internet Evangelist
Seriously: is there a cooler geek job than that?
Dean Kamen I did recognize on site. Double-checked his name-tag; sure enough. He's a little shorter than me and was not riding a Segway ;-)