Okay, so this post won't be nearly as dense or tome-y as Moby Dick. My brother suggested I post my endorsement for one of the atheism symbols floating about at Pharyngula. AA picked the "empty set" symbol while I went for this:
I like this symbol since it could be plausibly linked to something else if you find yourself in a situation hostile to atheists. I'd say it's the logo for the Airex corporation from Wipeout:Pure. It also kind of looks like a sleeping abstract question mark.
Of course, that sort of thing may not work out for you. I just kind of like the way it looks. Feels vaguely upbeat, which is one of the things I believe PZ was going for.