Well, we've got one

Congressman Pete Stark (D-CA) admits to being non-theist.
http://scienceblogs.com/pharyngula/2007/03/should_we_be_happy_about_this.php

http://www.issues2000.org/CA/Pete_Stark.htm
has his voting record, which looks pretty solidly liberal. I don't know why he voted against small business associations buying health insurance.