Hot on the tails of 20th Century Fox getting in trouble for their Silver Surfer marketing (placing Silver Surfer pics on the back of quarters), Warner Bros. followed it up with their own currency marketing at Comic Con, placing Joker faces on numerous one dollar bills along with the phrase "Why so serious?" (Here is an article about the other wacky things they did to promote Dark Knight at ComicCon).
Note that they specifically did NOT put the web address, www.whysoserious.com, so do you think that creates a good enough loophole? Click "Read the Rest" to see the pertaining federal statute and play lawyer yourself!!
US Federal Law
Â§ 475. Imitating obligations or securities; advertisements
Whoever designs, engraves, prints, makes, or executes, or utters, issues, distributes, circulates, or uses any business or professional card, notice, placard, circular, handbill, or advertisement in the likeness or similitude of any obligation or security of the United States issued under or authorized by any Act of Congress or writes, prints, or otherwise impresses upon or attaches to any such instrument, obligation, or security, or any coin of the United States, any business or professional card, notice, or advertisement, or any notice or advertisement whatever, shall be fined under this title.
I think by avoiding actually placing the web address on the bills, they successfully skirted violating the statute, but I'm curious as to what you other legal minds out there think. My pal Slam makes a good argument that, really, there is no way they can claim this ISN'T advertising, so yeah, Warner Bros. screwed up.