iPhone & iPod touch:
<link rel="stylesheet" media="only screen and (max-device-width: 480px)" href="https://stackoverflow.com/questions/3839809/iphone.css" type="text/css" />
iPhone 4 & iPod touch 4G:
<link rel="stylesheet" media="only screen and (-webkit-min-device-pixel-ratio: 2)" type="text/css" href="https://stackoverflow.com/questions/3839809/iphone4.css" />
iPad:
<link rel="stylesheet" media="only screen and (max-device-width: 1024px)" href="https://stackoverflow.com/questions/3839809/ipad.css" type="text/css" />