There are about five decent MMO's for the iDevice platform:
Parallel Kingdoms
Anrufen Online
Graal Classic Online
We Rule
Outer Empires
First off there is Parallel Kingdoms. Its a google maps overlayed with a fantasy world. At first release it was really simple and crashy but its far more stable and has tons of features such as cities, markets, lots of enemies, your own homes and defensive structures, friendly lists, etc.
There is also Anrufen Online, a Symbian port MMORPG. Looks similar to Diablo II. I have not tried it but it looks promising.
Another is Graal Online Classic, a Final Fantasy (I or II) style MMORPG. Simple to start but stable and has new features and more areas being added.
Also there is the upcoming We Rule from Ngmoco. Its a bit of a stretch for a normal MMO as this game is like a management game like farmville but instead you rule a kingdom. You can visit your friends kingdoms and work together. Its not released for another two or three weeks but it seems like its off to a solid start.
And of course the first real MMO for the iPhone, Outer Empires. It offers the deepest gameplay with a galaxy of 20,000 star systems. This game is cross platform, all the major browsers, facebook and the iPhone. Its a huge universe of 20,000 star systems that you can colonize, mine, make space stations, explore, research and produce advanced tech to sell on the galactic market to other players and of course there are factions that lead to a lot of space wars and politics. Its a very player driven MMO and hugely immersive. Since it is built with simplicity and lowbandwith in mind it plays well even over edge. Now heres the big problem, the iPhone app while having a great interface and low bandwidth use is a big battery hog and it is prone to often crashes.
If you use Outer Empires then please use both the iPhone version and browser, it will give you the best of both worlds. Some of the tedious tasks like mining or exploring can take forever when the app crashes but the app is much smoother for combat.
Visit outer-empires.com for more info or
AppianFrontiers.com (my multi Faction Commonwealth Alliance site)