I really like this idea...

Here is the link to makezine:

http://blog.makezine.com/archive/2010/02/envelope_made_from_google_map_of_re.html

It's an envelope, but inside is a map of the return address. I could imagine all sorts of other pictures going inside an envelope like this. But I imagine the receiver has a chance of severely damaging the 'art' (or in this case map) depending on their method of opening the letter.

If a letter opener is used, the map or art might very well go unnoticed.

Still, this is a very inspiring idea.