How does a mobile WordPress theme differ from a simple theme?

It’s really not a “wordpress” specific, it’s just the css. You just give it a css for mobile browsers. That’s it.

here are few good readings on how to get about the css for mobile:

• http://www.html5rocks.com/en/mobile/mobifying.html
• http://net.tutsplus.com/tutorials/html-css-techniques/responsive-web-design-a-visual-guide/

if you still have some questions, feel free to ask. 😉