I am developing a site with a mobile version and am trying to figure the appropriate way to manage the URLs for search engines. So far I've considered:

 1. Having a mobile site with
            rel="canonical" links to the regular
            site.
 2. Putting both the mobile
            site and full site on one URL, and
            doing user agent sniffing.
 3. Another
            opinion:

> Spencer: "If you have a mobile site at
> a separate location or URL, you should
> 301 redirect each and every mobile
> page to its corresponding page on your
> main website. Employ user agent
> detection so that the mobile optimized
> version is served up if someone's
> coming in from a hand-held.
> - http://developer.practicalecommerce.com/articles/1722-Mobile-site-Development-Best-Practices-for-SEO-Usability

Both 2 and 3 make it hard for a user who wants to switch to the full site or mobile site manually, but I'm not sure 1 is the best alternative.

What's the best way to write URLs for a mobile site?