Spacetime Studios has released an MMO for the iPhone and iPod Touch called Pocket Legends. The game is already available for the iPad. It's free to download and play, with Spacetime charging for additional expansion campaigns.
Pocket Legends is a 3D multiplayer action-RPG set in a cartoon fantasy universe which works over wi-fi, 3D or Edge connections.
According to Pocket Gamer, three additional campaigns are available to buy for $1.99 each. The game's interface has been cut back from the launch iPad version, but otherwise the content is identical.