so würde es wohl funktionieren:
iphone.css:
#computer { display: none; }
#iphone {display: inline; }
computer.css:
#iphone { display: none; }
#computer { display: inline; }
test.html:
<html>
<meta name="viewport" content="width=320"> <!-- gehört das hierher? -->
<head>
<title></title>
<link rel="stylesheet" href="computer.css" type="text/css">
<link rel="stylesheet" media="only screen and (max-device-width: 480px)" href="iphone.css" type="text/css">
</head>
<body>
<div id="computer">
Computer
</div>
<div id="iphone">
iPhone
</div>
</body>
</html>
viel Spass mit dem iPhone ;-)