WordPress.org

What functions are available?

The most useful ones are:

<?php 
is_handheld(); // any handheld device (phone, tablet, Nintendo)
is_mobile(); // any type of mobile phone (iPhone, Android, Nokia etc)
is_tablet(); // any tablet device (currently iPad, Galaxy Tab)
is_ios(); // any Apple device (iPhone, iPad, iPod)
?>

You can also use:

<?php 
is_iphone();
is_ipad();
is_ipod();
is_android();
is_blackberry();
is_opera_mobile();
is_palm();
is_symbian();
is_windows_mobile();
is_lg();
is_motorola();
is_nokia();
is_samsung();
is_samsung_galaxy_tab();
is_sony_ericsson();
is_nintendo();
?>

Do you have any examples?

Yup. This first example disables the sidebar for mobile/phone devices:

<?php 
if (!is_mobile()) {
    get_sidebar();
}
?>

This second example loads a specific stylesheet for Apple devices (iPhone, iPod and iPad):

<?php 
if (is_ios()) {
    wp_enqueue_style('ios', get_template_directory_uri() . '/ios.css');
}
?>