If you are looking for a plugin and would like to mess around with the installation, configuration, bug-tracking, updates etc, here are some of the solutions on the market:
Wapple Architect Mobile Plugin for WordPress is a plugin that allows you to mobilize your blog , it supports different themes, changing photo size display, navigation menus, remote admin configuration and Google Adsense and AdMob ad capability.
The WordPress Mobile Pack is a toolkit to help mobilize your WordPress site. It has a mobile switcher, themes, widgets, and mobile admin pane.
WPtouch will automatically transforms your WordPress blog into an iPhone application-style theme, complete with ajax-loading articles and effects when viewed from an iPhone.
The easiest and fastest solution on the market is the Mobilova Solution, it helps you create a mobile version of your site in less than 2 minutes and you don't even need to mess around with downloading stuff, installing things, administrative and bug tracking issues/updates. Try it now with the 30 Days free trial and pay later with your ad revenues, It's disruptively better and faster than any wordpress Plugin.