Fleck Touch from Self Aware Games has made its way from the iPad, to the iPhone. The game currently only supports the iPhone, and not the iPod touch, because of the iPod touch's low RAM. As well, the game does not currently work over cellular connections, requiring wifi connectivity.
This is a synchronous MMO game, with social elements. Players can build gardens and fight zombies. It's two mega-popular gaming trends all rolled up into one package! Players can design their home gardens in their own designs, and plant defenses to ward off zombies and other players. The game is based off of real-world maps, but it is not particularly location-based, as it's possible to go to any location and operate there. The game is currently locked in to the continental United States only, but this is because of ongoing work to expand the servers. According to Self Aware Games' Seppo Helava when I spoke with him at GDC, some users once tried to walk from the US to Madagascar. The servers did not handle this well. Hopefully soon, this dream will be realized. Until then, the iPhone update for Fleck Touch is available now.