Here are a lot of really funny ones: Mental Floss T-Shirts
My favorites have to be:
"Entropy (it ain't what it used to be)"
"Ambiguity: what happens in vagueness, stays in vagueness"
"Homonyms are a reel waist of thyme"
Besides those, I'm designing a T-shirt for my senior class (the calls of 2011). It is going to have an image of an amp dial turned up to 11 on the front, with the words "one louder" underneath it. On the back it's going to say "We Go To '11" and "PHS Seniors". Please tell me at least one of you has seen/heard of the movie "This Is Spinal Tap"?