I use one of my scripts to display a mobile version of my sites using the RSS feeds, but sometimes I have to redirect people automatically, so I usually use this piece of code somewhere:

$useragents = array ("iPhone","iPod","blackberry","palm","smartphone","iemobile");
$oniphone = false;
foreach ( $useragents as $useragent ) { if (eregi($useragent,$container)){ $oniphone = true; } }
if($oniphone){header("Location:/m/");exit;}

If I’m on a wordpress site, then the code is placed in my functions.php file for my theme. If I’m on another site, then I usually place it on a file I know will get called.

Generally, you can change it to look for a cookie if you choose to let people opt for the full version of your site, but otherwise, this code comes in handy.