Thanks to local screen printers and services like Cafe Press, it's pretty easy to get any slogan you want onto a T-shirt. I recently found a wiki at:
that is a collection of various bike-related slogans. A wiki is a publicly editibable page on the internet, kind of an electronic chalk board that anyone can edit. My favorite slogan on the list is:
This Vehicle Runs On Alternative Fuel (With A Picture Of A Donut Next To It)
I've never seen a shirt like that but I may have to spend a little time with some imaging software and make one up.
If you've got a great idea for a T-shirt slogan, stop on by the wiki and add your own.