Redirect mobile devices to alternate version of my site

media="handheld" doesn't work with modern smartphones like the iphone which pretend to be a desktop browser (uses the screen media type).

http://detectmobilebrowser.com/ Free & open source, has a comprehensive mobile user-agent checker available in many languages - javascript, php, asp.net, ruby, etc.

You could use a device description database (such as WURFL) which will recognise the client device from the request headers. You can then query that database to decide if the device can handle your site (e.g. support javascript, or is the screen big enough) before deciding whether to redirect them to a different site.

You don't mention your environment, but WURFL supplies APIs for Java and PHP, and maybe others as well. If there's no supplied API, you can still use WURFL, but you'll have to parse and process the XML data yourself.