One part of the show that is definitely aiming for the geeks is having guest stars like Wil Wheaton (who has played himself multiple times, as Sheldon's nemesis), Steve Wozniak, George Takei, and astronaut Mike Massimino.

One guest star Steven Molaro would still like to see on the show is Stephen Hawking.

http://geekout.blogs.cnn.com/2012/02/16/with-the-big-bang-theory-have-nerds-conquered-tv/

http://t3.gstatic.com/images?q=tbn:ANd9GcSwumntsR3WtFtIy6Gq4SH0kMD4IzG6HrH41D9m8irRAu8PcjgFHg

In keeping with George Takei, Wil Wheaton, and Levar Burton, I'd have to think one (or both) of Patrick Stewart or William Shatner.

Nimoy!!!!