Rattlesnake Island local post covers are sought after by some Cinderella collectors. Don't know if it's still done, but people would buy the local post stamps on the island and their mail would be flow out for delivery to a U.S. post office for mailing.
The website I mentioned does say it was restarting in 2005 and stamps seem to have been issued up to 2007. Nothing yet about 2008. I think I have another one or two someplace. Now if I can find my Atlantis cinderellas.