On the These Days blog there's a recent post talking about creating a mobile version of your site and how you can detect if the visitor is using a mobile browser or not using WURFL.

WURFL, The Wireless Universal Resource File (WURFL) is an open source project which collects information about all of the different mobile devices in use. It is constantly being updated, so as long as you keep your WURFL definitions up-to-date you don't have to worry about your detection scripts not recognising new devices. By querying a WURFL database with your visitor's User Agent string, you can not only determine whether the device they are using is a mobile device, but whether it has a touch screen, can make phone calls, is a tablet (iPad) and more.

They include a snippet of code that uses the Tera-WURFL service and a call to getCapabilitiesFromAgent to see what kind of client is making the requests and what it supports.