How hard is this? Are there any WP themes that have a mobile version automatically built in?
Also, how does the functionality work, is it just basically like "if browser = mobile browser, then use this other code"?
IF you are using WP…. you should check out WPTouch. It’s a plugin that handles mobile theming. I’ve used it on several projects, and it works fairly well.
As far as it being "hard"… impossible to say. WP touch works pretty well, and most of the other big CMS’s have equivalents. Or you could just do it yourself, which isn’t terribly difficult if you are a decent hacker and familiar with what’s out there (IE, Sencha Touch, jQuery Mobile, etc).
Im making one from scratch for a new project I coded form the ground up.
It fucking suck, but this helped: