Thanks for your comment, Nadir. Search engines start their crawls with the Internet Zone Files. Each site only gets one entry and if you're using ".com" or ".co.uk", chances are your site has already been indexed for your desktop homepage. So, if you're using something like m.site.com for your mobile site, search engines may not find your site and recognize it as mobile content. However using a .mobi is the first way to indicate to search engines that your site is made-for-mobile. We recommend that you maximize your "findability" but using all points of entry for mobile and that your .mobi should be your primary mobile site address for clarity to the user and of course for the reason I outlined above. For more information on mobile SEO, please read our Mobile SEO Best Practices paper at http://mobithinking.com/best-practices/mobile-seo-best-practices. Oh, and google.mobi works great :)

Thanks,
Amy