I just don't associate T-shirts with religion as strongly as you do, I think.
This might be because I see a lot of people wearing nerdy T-shirts, or T-shirts associated with various interests/groups, like DnD, heir fraternities, or professional groups (chemical engineering society, etc.). From that perspective, having a rationality T-shirt just falls into one of those categories, and is therefore OK. It's just another medium that some people with certain interests use to signal to each other.
Like I said: do you see it differently? Are most of the T-shirts with slogans you see people wearing religiously-related?
I guess there are huge geographical (and age-cohort) variations in this kind of stuff.
What are your best short witticisms, suitable for use on a t-shirt, bumper sticker, or similar location? Ideally something that might make someone reading it think, or get curious enough to ask about it. Simple in-group identification is fine too, though.
For context, therufs is spending today at the NC Maker Faire making t-shirts, and asked me for suggestions this morning. As I was still mostly asleep, I wasn't very helpful.