I enhanced @willtate code a little.
Note: If you need to create the icons quickly use this online generator: http://iconifier.net/
Add the default icon
$apple_icon = array(
'#tag' => 'link',
'#attributes' => array(
'href' => path_to_theme() .'/apple-touch-icon.png',
'rel' => 'apple-touch-icon-precomposed',
),
);
drupal_add_html_head($apple_icon, 'apple-touch-icon');
Loop through to add various sizes
$apple_icon_sizes = array(57,72,114,144);
foreach($apple_icon_sizes as $size){
$apple = array(
'#tag' => 'link',
'#attributes' => array(
'href' => path_to_theme().'/apple-touch-icon-'.$size.'x'.$size.'.png',
'rel' => 'apple-touch-icon-precomposed',
'sizes' => $size . 'x' . $size,
),
);
drupal_add_html_head($apple, 'apple-touch-icon-'.$size);
}
HTML Output
<link sizes="57x57" rel="apple-touch-icon-precomposed" href="sites/all/themes/yourtheme/apple-touch-icon-57x57.png"></link>
<link sizes="72x72" rel="apple-touch-icon-precomposed" href="sites/all/themes/yourtheme/apple-touch-icon-72x72.png"></link>
<link sizes="114x114" rel="apple-touch-icon-precomposed" href="sites/all/themes/yourtheme/apple-touch-icon-114x114.png"></link>
<link sizes="144x144" rel="apple-touch-icon-precomposed" href="sites/all/themes/yourtheme/apple-touch-icon-144x144.png"></link>
<link rel="apple-touch-icon-precomposed" href="sites/all/themes/yourtheme/apple-touch-icon.png"></link>