Thanks for all of these (and to the original poster, too). As it happens, I just started looking at editors for a mobile application.
Some instant thoughts:
Create seems to work fine on the iPad. Mercury sort of works, but kept switching out of edit mode for some reason. Aloha didn't seem to work at all.
As with some of the other commenters, I'm not wild about having the editor tied to a Rails backend (I didn't explore what kind of backend requirements Create has, though they seem to imply on the front page that it's at least somewhat portable).
The only other editor I've found that appears to work on the iPad out of the box is CKEditor. My old standby, TinyMCE, is supposed to support it, but I didn't have much luck (it may be that this isn't in the release yet -- I'll go back and explore the source tree later).
Granted, Apple has only had proper support for contenteditble on iOS for a couple of months, but I urge editor teams to please try to make their stuff work on the iPad. There's a huge unmet demand for that, and there's a first-mover advantage here. :-)
If they can do something clever with the UI to make the editor also usable on a device with a smaller screen (e.g. the iPhone), they'll really be cooking with gas.
Create actually is not a WYSIWYG editor, but instead provides the CMS framework around it (objects, persistence, workflows).
Currently we support Hallo (as used in the demo) and Aloha Editor, but I'd love to have Mercury there too. Take a look at the editable widget to see where it could be added:
Aloha uses jquery-based selectors to identify editable content areas as opposed to Create's more standardized and semantically accurate RDFa.
(This is based on all of 3 minutes of poking around so I don't claim complete accuracy.)