Ben Goldacre, who writes the Bad Science column in the Guardian, has a t-shirt for grown-ups saying 'MMR works - tell your friends'.
Am trying to convince him to do it for babies as well.
Conversely, my nephew has a t-shirt that says 'Why? Why? Why?' all over it - I want one!
Agree about the dreadful ones. My least favourite is 'I like boobies'.
Ooh, what else.... a friend's MIL got her one from the Royal Opera House for her little girl that said 'Diva in training', but fortunately she was too big for it.