Drupal doesn't have a built-in support for mobile and handheld devices (PDAs). If you want to make your site more accessible to mobile users, your only option now is to have an additional "mobile" theme in your set-up, which would be very simple and lean, and somehow use it automatically when users access your site from small devices.

I have just seen new Dell's Axim PDA, version X51v-624. I am actually quite impressed, since it has almost everyting I want from a PDA: Windows Mobile 5.0, a lot of RAM, VGA screen, powerfull CPU and long-lasting library (if you buy the double-capacity one).