Seems that Iranian activists are using banknotes to relay messages:
http://payvand.com/blog/blog/2009/11/16/exhibit-iranian-banknotes-uprising/
Make sure to check it out and see all the examples of the modified banknotes.

From the post:
'Anti-government activists are not allowed to express themselves in Iranian media, so theses activists have taken their expressions to another high circulation mass-medium, banknotes. The Central Bank of Iran has tried to take these banknotes out of circulation, but there are just too many of them, and gave up. For the activists’ people it’s a way of saying “We are here, and the green movement is going on”.'

Lets see what we get when we analyze this from a data communications perspective:

Currency serves as the content container, or "packet". Each "hop" is defined by the act of monetary exchange between two people. This is a "delay tolerant network", or DTN - there is no need to set up a synchronous end-to-end connection, and information propagates opportunistically from node to node. Another networking term that comes to mind is "piggybacking" - when extra payload is attached to packets that are going to be sent anyways, like a hitchhiker going for the ride.

The central bank system can act as a firewall or filter node that may remove unwanted messages, or as a "packet sink" where all packets end up.

The originator of the message, or "sender", might be able to plausibly deny that he or she actually created the message on the bill. They could always say that they got it from someone else, making themselves seem like a relay node rather than the source. Of course, distributing too many of these to a single merchant might lead to suspicion... After the first exchange of a marked bill, every subsequent person actually is just a carrier and forwarder of the information. This somewhat analogous to the TOR (http://www.torproject.org/overview.html) style of anonymization.

But here's the interesting bit - Who is the destination, and what is the actual message relayed?

Different ones may come to mind, and in some way, all are probably valid: The destination might be other activists, and the goal is to show them that they are not alone and there are still others like them. Alternatively, the goal might to reach those who are still not active enough, and push them to action. In these contexts, the communication is not a conventional broadcast from one-to-many, but a chain of anonymous messages, passed on from person to person on a one-to-one basis. We get trails of information passing from one person to another, and each person has the choice weather to pay it forward, or keep it as a souvenir and break the chain...

Eventually, these packets would go through the hub of the Central Bank, which might be considered a destination on its own merit: The destination might be the government, and the goal is to show them the resistance is still out there, and perhaps growing. In that case, its not about the individual messages, but about the aggregated effect of the bulk - just like a distributed denial of service (DDoS) attack, where many packets are sent from many destinations, all headed into a single end point. There is also something rebellious in the essence of the act of defacing the banknote, which is a federal offense in most countries that I know of.

In his talk at the Call For Action seminar that Prof. Chris Csikszentmihalyi and I organized last January, Ethan Zuckerman of the Berkman Center mentioned a successful approach for online media activists: If the message or activism act is blended into a critical service, or one that is heavily used by many people, it is hard for a government to block that service completely. They could block specific activist websites, but it is harder to block a service like Facebook, Twitter, or Yahoo in their entirety because of the backlash. This currency messaging system is very similar - as the original post mentions, "there are just too many of them" so it gave up trying to remove the marked bills from circulation.

This banknote image at the top (and at http://payvand.com/blog/files/2009/11/iranian-banknotes-green-movement12...) is quite interesting - as this bill passed through the chain of forwarders, its content was modified and accumulated. First the message "Death to Khamenei" was marked on the banknote, then it was crossed out by another relayer, and then a third relayer added the response "Shame can not be erased by crossing out". There is something romantic about using an analog rather than digital packet that is tangible and also keeps memory of its own past.

Finally, we can see Marshall McLuhan's immortalized expression come to life, since the medium really IS the message. It’s not about the specific subversive content at all, but about the existence of the modified bill, and at the same time the existence of its originator. It is all about the senders broadcasting their defiance and their presence. In networking this would be considered a "ping", a "keep-alive", or a "heartbeat". With the way activists are treated these days, the last two terms carry quite a literal meaning.