How about a Co-founders Search Site?
Serene: A few more sites that help  to find co-founders:http://founderdating.com/
http://startupsquare.com/Why don't you start a meetup group like this one? 
http://www.meetup.com/Co-Founders-Wanted-Meetup/
Briefings from the meetings:
http://aurametrix.blogspot.com/2010/03/bay-area-startups-loo...
http://aurametrix.blogspot.com/2009/11/bay-area-startups-loo...If you want to build something similar to what already exists, you will be able to do it by yourself.
If you want to create something like e-Harmony - matching people based on demographic, educational, job history, psychological and behavioral characteristics and their encounters, feeding gigabytes of data into sophisticated models on a daily basis, think about measurable expectations before proceeding.