Spy Box, the lifecasting postal parcel
Inside an innocuous parcel sits a camera, peering out of a hole to snap images every ten seconds as it is shipped through the postal system. It's the "Spy Box", a project by UK artist Tim Knowles. The resulting 6,994 images were edited together to make a movie which was displayed alongside the Spy Box in the gallery to which the parcel was shipped.
If the Spy Box were to be sent through the American postal system, its images would almost certainly be in violation of some rule or restriction. (Not that it would make it through without being sent to the bomb disposal hopper.)
There's a whole discipline within pop art that uses the postal system as its conduit, from attempts to mail strangely shaped items to shows that make the mailbag as important as the art.
Project Page [TimKnowles.co.uk via Make:]

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I would disagree with whether or not this is 'more illegal' in the US... for starters, I did it myself, sort of, many times between 2003-2006.
Regard!
A little more here (by other people).
The project seems to be to an upgrade of Janek Schaefer's 1995 project 'Recorded Delivery', where the artist put a voice-activated recorder through the London postal system. Okay, this project is much cooler, has a nifty box, etc, but props to Janek for doing it first. And giving it a better name.
KVH: First of all: awesome. That's a great project.
Second of all, while I'm totally just guessing, taking pictures or video of the inner workings of the postal service seems exactly the sort of thing that our paranoid government might be unhappy about. Perhaps I'm wrong!
I think I need to rig up a similar device for my checked luggage.
Certain airports (esp. Seattle) seem to have fat fingered TSA and/or baggage handling personnel. Getting justice against these thieves is a laugh. It would be nice to have the last laugh next time they decide to steal from someone's bag.
Of course TSA would hate this but, whatever...
Legality aside, if I worked in the postal system, I don't think I'd be too happy if some jackass invaded my privacy with one of these things and posted my picture online.
Well, for the record, my project was strictly voluntary, and required direct interaction by the postal employees to work... That said, on more than one occasion official letters and even the Postmaster General for whatever area the destination was, personally requested that we stop this project.
It's been on hiatus for a while -- I should fire it back up again....
And there is of course not only mail art, but mail science:
http://improbable.com/airchives/paperair/volume6/v6i4/postal-6-4.html