TheBabyWearer.com did this awhile ago and now have a cafepress shop. they let thier members come up with slogans and pictures. Are you just wanting the slogan or graphics as well? Checkout http://www.cafepress.com/thebabywearer
for thier selection and maybe some ideas? I made the "Babywearing for our Future" one with the graphic. Is there gonna be a prize for the winning slogans? (hehe)
I'll start rackign my brain for another fun design and slogan! Always fun!