Thanks, Joe-- but it isn't very funny (to me) if the word "pi" is written out. [The T-shirt also had "pi carried out to umpteen hundred decimal places on the back.)

So I'll just say the sticker said:

Got Milk?
Need
[Greek symbol for Pi]

Let's try holding down the "option" key on the Mac and hitting "p" [which yields the Greek symbol on my screen] and see what happens when I post it: