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. Since automatic detection is required and you probably don't want to change core Drupal files, you can use the new module Mobit - it handles the selection for you. I haven't tried it yet, but according to the bug tracker on the project, it lacks the .info file, so you'll have to add one before the module will be actually visible in the modules section of the Drupal administration section.