So, something I have noticed: The Google search page just doesn't look the same here with iPhone user agent as it does on the real thing. (Note fancy buttons on top for Reader, Calendar and Gmail). Presumably, these are CSS changes applied within the iPhone's browser rather than by Google themselves. Has anyone achieved this effect by changing userContent.css?

As for reverting user agent the clean way, you can manually edit ~/.mozilla/microb/prefs.js and delete the line with user_pref("general.useragent.override", ...