Guest suggestion - Phil Plait
aka the Bad Astronomer from http://www.badastronomy.com/. I thought of this while listening to one of the earlier shows when you were arguing with a creationist and cosmology/astronomy came up and you mentioned that none of you guys are really specialists in astronomy. He's got quite the following of scientific, freethinker type people that could substantially increase your membership. He's pretty openly atheistic and is fairly brutal in some of his blog entries about creationism and any other anti-science sentiments.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
- Eric Idle, from The Galaxy Song