Only one I can think of is Pocket Legends.
I like Pocket Legends.
Unfortunately, you can't (yet) have guilds, but all in all it's a nice game.
Not comparable to WoW, but you can see that they are extending the possibilies nearly every day.
Highland Warrior is a much more pc like 3d mmorpg on the ipad and iphone, it'll have things like clans and a much more realistic style. The game has been renamed to Celtic Heroes, and there are some images, video and more information at:
Official website for Celtic Heroes 3d MMO for ipad iphone ipod
I don't think it is completed yet. At least, that is what I got when I went to their website.
looks like they are acceptint requests to be beta testers :)
Not sure if these are MMOs but they are RPGs and they are on sale today...and they are pretty popular.
Battle for Wesnoth HD - $0.99
Battleheart - $0.99
Dungeon Defenders - $0.99
Duel: Blade & Magic XD - FREE (down from $8.99)
Dungeon Hunter HD - $0.99
Here is the list:
Games on the iPad - iPad game reviews, deals and more!
Touch Arcade has very active forums.
Worth checking there as they don't pull their punches.
Aralon HD and Sacred Odyssey are both good RPGs, but not MMOs.