Assuming there is no law against defacing your money (there probably is for fly posting) just write "bitcoin.org" on all the debt notes that pass through your hands then everyone who uses the money will see it.
Thanks a bunch for the ingenious guerrilla marketing idea. I've just gone ahead and done this for all the euro notes currently in my wallet. Here's photographic evidence as a sample:
I experimented a bit with different writing instruments, and in the end would recommend using a pencil. It works and looks better (at least on euro notes) than a ballpoint pen, doesn't permeate to the other side of the note as a felt tip pen would, and is erasable in case the payee gives you any grief about your illicit graffiti
Yeah, the erasability is important. While it's not illegal to write on Euro notes, notes that have been written on are no longer legal tender, so it's good idea to be able to erase that should someone not want to accept your notes.
The Conspiracy wants you, children. Oh, it wants you bad.
After I first saw "They Live" in late 1990, I had a rubber stamp made reading "THIS IS YOUR GOD" in big block letters of a size suitable for US banknotes. I'd stamp every one of the things that came through my hands for a year or two after that.
Nowadays I walk into a Tesco in Slovakia and buy something upstairs with a €10 note, getting a fiver back plus some coins. Unbeknownst to me, the five-euro note has a small tear on one corner. Take that note downstairs in the same Tesco, right afterwards, and try to pay for something and they'll refuse to take it. Sorry, you'll have to go to the bank ---and show ID--- to get the bloody thing replaced. Useless fucks.
So, my thought for this little bit of guerrilla marketing: Get a *small* rubberstamp made reading "bitcoin.org". Get some blue ink and dilute it down to something approaching the background of the €5 note. Stamp in an area where the mark won't be obtrusive, and not near the major security features. Make piles of them. Plenty will pass with no scrutiny at merchants, and at the end of the day you can always get a banked friend to take the pile you got tired of to his/her bank and ask for replacements, complaining that they've been collecting in the shop till.