The other day we got a $5 bill back in change, and noticed a big purple numeral 5 on the back. Sunam and I debated for several minutes whether or not it was real currency, then launched into a lively discussion of how cool the differnt Euro designs are, compared to US currency.
At work someone sent me this link http://www.typography.com/ask/showBlog.php?blogID=93 so I guess it was a real $5 bill.