I have just came across very nice article from Ewan MacLeod about The Mobile Web and the ‘m.’ solution. I agree with almost everything in the article and if you are interested in Mobile Web, I recommend you to read it.
Interesting is also a debate in comments, about what approach to use: m.something.com or something.mobi. I think both approaches will coexists together. Big sites will use "m.", because they are afraid what .mobi guys can do one day (.com domain have better rules/legislation), but smaller sites and projects will be fine with .mobi domain. We at Wirenode have several of them and are quite happy.