You might have heard of the markup language call WML (Wireless Markup Language) that was specifically designed for mobile devices in the early days. You can learn more about WML in Sitepoint's 2001 article on how to create a WAP site -- a WAP site being a site based on the Wireless Application Protocol.

Now today in 2008, you need not worry too much about WML, because mobile devices have advanced to a state that it you simply use your standard XHTML markup.

Although this is a variant flavor called XHTML-MP (where the MP stands for Mobile Profile) that uses fewer elements and has some other restrictions.

One major problem with designing pages for mobile devices is that there are such large variety of screen resolutions and aspect ratios.

Although you can validate your mobile device markup at W3C's mobile checker and run other software checks such as the ones found at ready.mobi and run them through emulators (like at mtld.mobi and developer.openwave.com) , there is no substitute for testing on ready phones. Make sure you test on a good variety of them.

Sitepoint has a great article for designing for mobile devices.