Auto Mobile Theme Switcher can change the theme to be displayed according to the detected browser automatically.

MobilePress is a plugin for WordPress that renders a mobile friendly version of your WordPress website or blog.

This plugin will detect mobile phones, and redirect to Mippin.com to display a mobile optimized version of the page.

Requires at least 2.0.2

This plugin is now deprecated, please check out the new mobile wordpress plugin at http://wordpress.org/extend/plugins/wordpress-mobile-pack/