I know the native app exists already, but I've got a great Mahjong game for the iPod Touch that you can play on the web, which is actually better than the native app.
Check it out if you want:
http://www.playtouchgames.com/games/mahjong/
Dan
Check it out if you want:
http://www.playtouchgames.com/games/mahjong/
Dan