Ideally, what you would do is write out an e-mail to a friend, then click the Google Envelopes button at the bottom of your text window. It would serve as an extension to your Gmail account (and it assumes you have entered your friend's mailing address).
"For a small fee" the program would print out your e-mail, fold it up, and insert it into an envelope that had been printed with a map of the distance between the sender and the recipient. Obviously, it would have to turn the map around most of the time, in order to make the sender's location always appear in the upper left.
Don't read the comments of the article in the link I provided, though. "Don't read the comments" is generally good advice wherever you read news on the Net, but it merits being mentioned in particular. I'll sum up: a bunch of people are very upset about the amount of paper that would be "wasted" on this project. Yet I doubt every single one of them is dedicated to recycling and mass transit, or even growing their own garden and storing solar/wind energy. They're just very judgmental of other people.
Personally, I thought this would have been a cute, fun idea to encourage people to send actual letters to each other. Yes, I hold the postal system to be particularly darling and I vaunt a little letter-writing above complete and total reliance upon online communication. And it's no secret I'm enamored of Google products, so I'm disappointed on a number of levels that this service never manifested.
P.S. Sorry for the lack of entries. Both my tower computer and my laptop crashed last week and I've been getting them examined. The tower is back home and running swell, but the laptop issued smoke out its rear ports and is pretty much cashed.