I kind of like the idea behind snOwned, a new wintry app on the App Store that claims to be a "multiuser, online snowball fight." I think there is some great multiplayer gaming to be had with the iPhone, and, if enough people are interested, there might be some in snOwned.
You download the app from the App Store (it's 99 cents, which is cheap but maybe not cheap enough for what it is), and then sign your email up along with a username. Once you've activated, you can then "throw snowballs" (which consists of three little fun minigame steps) at anyone else with the app installed on their iPhone or iPod touch around the world. And they can throw them back at you, you can play a short game to defend, stats are kept on the site, and so on. As with most social games, it all depends on who's in the game with you -- if you've got friends along to keep up rivalries with, it could be fun. But just throwing random snowballs at random people around the globe, as I did (my ingame name is "toucansamurai") isn't really worth the dollar's admission.
Still, it's a start -- the game can also track your location when you throw, so you can see where your snowballs are going on the globe, and who's throwing at you. I think there will eventually be a runaway game of this kind, where we're all playing with each other socially on our own phones in different places all over the world (Loot Wars is another game that's interesting in this vein, even if it's not quite there yet). And snOwned is another step closer to that. But as any MMO company will tell you, the only way to make a social game great is to have everyone playing it, and I'm not sure there's enough going on in this one to pull in a lot of really interested players.