I have a few, but most of them require explanations.
1) I'm in the Air Force, we wear Camoflauge BDUs as our day-to-day work uniforms. I recently saw a Cammo T-shirt with the caption "You can't see me".
2) The shirt I used to wear when riding my motorcycle, said "To be old and wise you must first be young and stupid."
3) Of course, there's the "make 7-up yours" shirts, that say "make 7" on the front, and "up yours" on the back!
4) For any anime fans out there, I have a T-shirt with Rick Hunter (from Robotech) in the "Uncle Sam" pose which says "We need YOU for the RDF!"
5) A great one for military members is by 7.62 design. It shows a Army soldier in full combat gear, aiming an M-16, the caption says "Final Notice, Payment Due"
6) Of course, for the conspiracy theorist out there, there's the BATF storm trooper, dressed all in black swat gear, BATF across the chest, holding an AR-15. The caption says "I'm from the government. I'm here to help you."