I've been looking for a good outlining app for the iPad and thought I'd document what I found here. I've used SoundNote which lets you record audio and take notes at the same time (Evernote also has this feature), iBrainstorm, Evernote and the standard Apple Notes applications. I've also heard really good things about Note Taker HD. They all have their uses but if you want to outline a subject, none of them are really setup for that.
For me, efficient hierarchical list keeping with a good interface and presentation is the primary requirement. It would be nice to sync this between devices or at least store the document on DropBox.
There are a number of mind mapping apps for the iPad which I won't attempt to review here but iThoughtsHD seems to be the leading one in that category. There are also numerous ToDo app solutions which sometimes providing outlining features. I'm a big fan of ToodleDo which provides a cloud solution that integrates iOS and Android devices aswell as a good web interface.
This looks like the most mature (if you can say that for a platform that's only a year old) of the outlining apps I've run across. It includes the ability to create outlines as tasks or lists and if you mark an outline as a task, it creates a checkbox next to each item to track it's completion. There's a cloud function called Outliner Online that allows you to sync your outlines and to share them with other users. A quick features summary with screen shots can be found at the CarbonFin website.
The combined reviews for the free and paid version of this app are mixed at best. Numerous comments about the user interface being difficult to use.