A List Apart has a great article about conditionally using different style sheets for mobile devices:
http://www.alistapart.com/articles/putyourcontentinmypocket/
You can also check out Apples Documentation on designing web content for the iPhone, although it's iPhone specific, it pertains to mobile devices in general:
http://developer.apple.com/iphone/library/documentation/AppleApplications/Reference/SafariWebContent/OptimizingforSafarioniPhone/OptimizingforSafarioniPhone.html#//apple_ref/doc/uid/TP40006517-SW1