There are a bunch of WordPress plugins that claim to make your blog mobile friendly but after looking through a couple of them the WP-PDA plugin is the best. This plugin support multiple themes such as an iPhone theme and a PDA theme. The cool thing about this is the iPhone and Touch display things completely different than the typical PDA browser so this makes it easy to modify content specified on these two main mobile groups. You can download WP-PDA here. Trust me when I say that iPhone and PDA users that read your blog will appreciate this plugin.
In the admin of WP-PDA you can specify what browsers will use the mobile theme easily. Just visit the plugins link on the top right and you will see a link near plugin editor that is titled PDA. Just update the list and click submit for changes to take affect.
One thing to note is you will need to add your Google Analytics code to the footer of the iphone-theme and the pda-theme for those users to be tracked by Google Analytics. I have also posted an article relating to mobile Adsense ads with this theme so if you want ads check out this link.