Sorry to be so pessimistic on this one, but I don't see much benefit. Kind of like .com vs .org. How much does that really help the web visitor?
I don't think this will do what we hope it will do, i.e. make things simpler for the mobile web user to find sites. And it's not really going to help us get nice mobile sites. Like Hacker says, many people will have different approaches to "mobile" versions. I'm afraid many or most will be pretty bad. Plus you won't know if the site is even aimed at phones or tablets or pdas or what.
To get to a nice mobile site you pretty much need to know the specific url in advance anyway. So the only thing you gain is a tiny little bit of consistency, like there will probably be a wsj.com and a wsj.mobi. Oh yeah, and you'll still have to pay for that one, for example, anyway!
Am I missing something and gone pessimistic for no reason? What's the benefit here, other than it's maybe easier to remember
www.cnn.mobi than
www.cnn.com/mobile or
www.cnn.com/avantgo or whatever it is that they use? Surely there's more to it than that!?