WSJ piece about mobile browsing ... It speaks about .mobi quite a bit then accurately points out: "Sites that end in dot-com or dot-net can also be designed so they show mobile-specific content when consumers access them through a mobile device."
How does that happen? The site reads the "user agent" as a mobile browser and then serves user-agent sized content. I wanted to show you this picture of address bar in a mobile browser. It looks to me like there is room for about 14 characters plus a 3 character .com/.net extension (18 characters without www. in front). So you should take this cue and keep your names under 18 characters.
There is also some talk in the piece about sub-sites such as rufkins.wap.com where you register a third level domain at wap.com. I don't not think smart companies will go for that.
Initially a handful of folks will build .mobi sites and play around with that ext. Watch what happens as the flood of the existing World starts making mobile friendly websites.. They will absolutely make it so their .com resolves in these little browsers. Why? So that they don't loose visitors to the mobile company's error search ... you can bet the farm on that.