The .mobi domain is about two years old but still not really popular. However surfing from a cellphone or PDA is getting more and more popular. Certainly since the launch of the iPhone.
Contrary to what some people might have expected (or hoped), it looks like the iPhone means the downfall of .mobi. The user-friendly browser in such an iPhone ensures that an ordinary website is displayed perfectly on a small device like a PDA. On Windows Mobile, the latest version of Opera Mobile does this at least as good.
It turns out that, even before everyone knows about a mobile browser, the software became so good is that there is no need anymore for a website especially designed for this.