Whatley at SMS Text News has written a good post about naming conventions for mobile sites. He comes down on the '.m' side, meaning his preferred convention is that you replace the 'www' in any URL with 'm' to get the mobile version of a website (e.g. www.yahoo.com becomes m.yahoo.com if you are looking for mobile content). I have to agree, and this is exactly what I did with the Nelso mobile site.
The other popular option is to use the '.mobi' top level domain, and give your site an URL like www.yahoo.mobi. This is a poor solution for a number of reasons, but one of the biggest objections I have is that it assumes that a website has only one mobile version. What to do about sites like Nelso that have content in many languages, and use country-level domains for this content? Where does the Czech user go to access our mobile content? I guess you could send him or her to the .mobi site, and then present a splash screen to choose language and location, but that feels a lot clumsier than just using a domain like m.nelso.cz for the Czech mobile site.
I'm going to stick with the '.m' naming convention. It'd be great if this convention became universal, and I could count on the Flickr mobile site being at http://m.flickr.com (it is, by the way). Having a TLD like '.mobi' makes no real sense. Should I offer FTP access to Nelso at www.nelso.ftp? I think subdomains will work fine.
From SMS Text News via the Wirenode Blog