The $wgAppleTouchIcon config var can specify the path to an icon to be used for home-screen bookmarks on iOS devices.
Originally there was only a single resolution required, 57x57 pixels for the classic iPhone and iPod touch, so only a single link was needed.
Newer devices with larger or sharper screens prefer other resolutions: 72x72 for iPad, and 114x114 for the iPhone 4 and newer iPod Touch models with high-resolution displays. Under iOS 4.2, CSS3 multiple sizes may be specified by providing multiple links, with the 'sizes' attribute listing width & height.
Recommend allowing $wgAppleTouchIcon to be an array, perhaps:
$wgAppleTouchIcon = array( 'default' => '/iphone-57.png', '72x72' => '/iphone-72.png', '114x114' => '/iphone-114.png', );
or a more device-name-friendly:
$wgAppleTouchIcon = array( 'iPhone' => '/iphone-57.png', 'iPad' => '/iphone-72.png', 'iPhone 4' => '/iphone-114.png', );