Shadow Cities is an interesting title from Finnish developer Gray Area. It's a location-based MMO, in that you level up a character while fighting with other players and claiming territory, all of which is based on the real world around you and your iPhone. The game allows you to cast spells with special gestures, which are resource driven and set up to play long campaigns in small bursts of time -- it's designed perfectly for the iPhone and its various interfaces. I found the game a little confusing when I saw it at GDC, but that was an earlier version. Even if the game is a little complicated, it's rewarding when you figure out exactly what's going on.
And it's even more rewarding when you have fellow players around you -- if you're in a city such as New York or San Francisco, you'll probably be in luck finding some competition in the local area. Everyone else will still have neutral forces to fight against, but the game's more fun when you participate in the overall PvP campaigns. You can "beacon jump" to other cities, but things are much more fun when you're fighting for a home base.
At any rate, Shadow Cities is currently free in the App Store, and at that price, I can't help but recommend you check it out. A lot of people have tried location-based MMOs already (and I bet we'll see even more of these in the future), and while Shadow Cities isn't exactly the easiest game to get into, it does offer some good innovation in the quickly growing genre.