Howtoforge posted a tutorial about redirecting users to mobile or normal website based on device using mod_rewrite

Since the massive rise of smartphones and tablets like the iPhone, iPad, Android phones and tablets, BlackBerries, etc. you might have considered creating a mobile version of your web site. This tutorial explains how to configure Apache to serve the mobile version of your web site if the visitor uses a mobile device, and the normal version if the visitor uses a normal desktop PC. This can be achieved with Apache's rewrite module.

  Apache2: How To Redirect Users To Mobile Or Normal Web Site Based On Device Using mod_rewrite