Is it illegal to write on a dollar bill

Is it illegal to write on a dollar bill? In my opinion the law says that you cannot deface a banknote if your intent is to render it unfit for circulation (or advertise on it), so basically if you intentionally do something that makes it unusable in a vending machine you are breaking the law.

Title 18, Section 333 of the United States Code, is the Actual law pertaining to the topic from which the above opinion was developed.

DEFACEMENT OF CURRENCY:

Defacement of currency is a violation of Title 18, Section 333 of the United States Code. Under this provision, currency defacement is generally defined as follows: Whoever mutilates, cuts, disfigures, perforates, unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, Federal Reserve Bank, or Federal Reserve System, with intent to render such item(s) unfit to be reissued, shall be fined under this title or imprisoned not more than six months, or both.

The above law doesn’t say anything about 'writing' on a dollar bill, but they are also ambiguous terms and it says you can’t do those things with intent to render such item(s) unfit to be reissued. So you are not going to tear a dollar bill, because you could not use it and you'd be out a buck, but a terrorist might want to destroy money for some nefarious purpose. You might however want to write something on it like a phone number, but your intent probably would not be to render it unfit to be spent as you'd be out a buck.
So what purpose might writing on a dollar be considered a crime? If a business bought a lot of one dollar bills and printed a logo or web address: 18 USC Section 475 regards the attachment of advertisement to legal tender.
 
What about having a dollar signed by a celebrity? The United States Department of Treasury’s Bureau of Printing and Engraving discourages writing on money but has had spokes persons and fact sheets that make statements saying it is not necessarily a crime for example to have a celebrity sign a dollar bill. This could be because the bill is not meant to be placed back in circulation, since it would most likely be saved as a celebrity autograph souvenir as well as there would be no intent to make it unusable. Barack Obama and Hillary Clinton have refused to sign dollar bills, but Bill Clinton, George Bush and some Treasury Secretaries have signed them as well as many many celebrities(check Ebay) .

So is the Where's George and similar websites breaking the law? Probably not as they state "we do not encourage the defacement of US Currency" and do not seem to be the one's actually stamping the bills "We no longer sell any Where's George? rubber stamps.". Also since Wheresgeorge.com has received an estimated 244,300 visits over the last month, if they were breaking the law, you would assume the Secret Service would have already stepped in at some point since the site was officially launched in December 1998.

What about squishing a penny in those souvenir machines? A fraudulent intent is required for the criminal violation, so the act of compressing coins into souvenirs does not appear to be illegal without other factors existing. Since 'you' are making an impression on the coin and a business is not impressing or attaching a business or card or advertisement to the coin no laws seem to be being broke.

By the way if you are a celebrity and want to sign and send me a dollar (or Hundred dollar bill, since I'm not going to tell you what you can or can not write on) you can mail it to PO Box 459, Tolleson, AZ 85353.

Sources:
-FoxNews.com - By Kimberly Schwandt - Published July 05, 2012 - Oak, Harbor, Ohio - President Obama turned down the offer to sign a dollar bill, citing it as a federal offense to write on currency.
-Politifact.com - By Bill Adair - December 19th, 2007 - Des Moines, Iowa - Hillary Clinton was asked by a shopper to autograph a dollar bill. She refused saying it is illegal to sign money.
- Encyclopedia of the Modern Elongated - by Angelo Rosato - Reproduced letter from the Department of the Treasury to Mr. Vance Fowler.
- wheresgeorge.com/faq-rules-privacy.php
- http://www.moneyfactory.gov/historicallegislation.html

United States Secret Service is the department tasked with enforcement of defacement of currency, they are also the agency that deals with counter fit currency.

*I am not a lawyer, above is my opinion on the law and should not be taken as legal advice.