No, they're not teenage mangos.
David Karp, the "Fruit Detective", and author of an assortment of great fruit articles, has a piece out in the New York Times on this realitive rarity. I don't know how long non-subscriber access will last, but for now you can read it here.
Update: Okay, so I moved too slowly. All you non-New York Times subscribing types (I really ought to subscribe, considering I buy it more days than not) can turn to a life of piracy and read an illicit copy here, sans photos, for as long as it lasts.
Update 2: For good measure, here's another earlier non-Karp New York Times piece on mangosteens, no doubt questionably reproduced as well.
Update 3: And because David was nice enough to contribute them, a rare treat here on the Fruit Blog: in-line photos! The idea of the "Photo of the Day" thing was to limit hosting requirements, but since that issue has been solved, there's really no need to be so restrained, so I'll put one here and one over in the sidebar. (These are copyrighted by David Karp, so please don't use them without permission). So, without further delay...mangosteen!:
Update 4: Oops...still getting the hang of the photo thing. That should fix it.