This script will redirect the visitor to a folder named mobile in the root of your host server. Create the dir, copy your mobile site there, add this code somewhere in the body of your main home landing page, and it should redirect to the files in /mobile dir.

Code:
<script type='text/javascript'>
<!--

if (document.cookie.match(/(^|;)\s*is_mobile=1/)) {
	document.write(
		"&nbsp;&nbsp;&nbsp;&nbsp;" +
		"<a href='?mobile'>Mobile Site</a>"
	);
}

//-->
</script>