If you want to apply CSS styles only for the iPad, you can do
(at least at this moment) in the following way:
<style type="text/css" media="only screen and (min-device-width: 768px) and (max-device-width: 1024px)">
body {
text-align: center;
background: #434242 url(img/_bg_iphone.png) no-repeat 50% 10px ;
}
#wrapper {
position: relative;
margin: 170px auto 50px auto;
width: 320px;
height: 460px;
}
</style>