The Many Colors of Urine Webpage
Blue or green urine may be caused by a blue dye such as methylene blue, a component in several medications (Trac Tabs, Urised, Uroblue) used to reduce symptoms of cystitis. A blue pigment may also be produced by Pseudomonas infection
Wasn't this in an episode of M*A*S*H or something? We've all searched for something of this sort -- what a prank it would make!