JavaScriptを使ってUAでiPhone判定ができます。
<script type="text/javascript">
if (navigator.userAgent.indexOf('iPhone') != -1) {
document.write('<link rel="stylesheet" type="text/css" href="./layout/iphone.css">');
}else {
document.write('<link rel="stylesheet" type="text/css" href="./layout/pc.css">');
}
</script>
if (navigator.userAgent.indexOf('iPhone') != -1) {
document.write('<link rel="stylesheet" type="text/css" href="./layout/iphone.css">');
}else {
document.write('<link rel="stylesheet" type="text/css" href="./layout/pc.css">');
}
</script>