In fact this mobile issue is terribly complex...
I used at least 4 hours today to test some of the most famous mobile browsers / emulators and can conform that mobile Safari and Opera can't use editor properly with any mobile model or mobile operating system I tested - these browsers simply can't use contentEditable. You can test mobile opera online with Opera Mini Simulator - a live demo of Opera Mini 5 that functions as it would when installed on a handset in http://www.opera.com/mobile/demo/ . Buttons are shown as expected but you can't use selection etc which makes modifying of content impossible. Therefore a better option is to render plain textarea and Opera has a cool keyboard for that purpose...
On the other hand latest models of Nokia (I tested N900 and N8) use a browser that is based on Mozilla but moodle (moodlelelib.php) does not detect it as supported browser for editor. Still on the main site of Moxiecode I was able to use both buttons and contentarea of editor - yet it was far from perfect user experience... attached an image from N8 emulator http://apu.ndhub.net/devices
Another, much more difficult thing to handle is the (missing) core theme support for mobiles...