OK, you really can't use it for tracking meeting spend (sorry!). But when my dad sent me a link to WheresGeorge.com, I had to check it out. The site lets you enter the serial number of any bill in your wallet, and in theory anyway, you'll be able to see where it came from and where it goes after it leaves your hands. I think it might need a much wider adoption, seeing as there are who-knows-how-many U.S. bills in circulation—the pitiful few I had on hand to test it out didn't have a history yet.
But what a cool concept.