I'm a fan of just developing your .com to fit mobile sites.
My two reasons are:
1. If the user is on their phone & typing in the URL, they probably are going to do .com instead of .mobi. And if they're using a search engine, they may not find the .mobi. Unless, of course, you are doing some kind of autoredirect that detects when people are on mobile devices and then sends them to the .mobi site.
2. From a content management perspective, I like the simplicity of only having one site to update. I suppose this really depends on how much content you have on your site, how often you update, and how your back-end works. I'd look at how much time and effort it takes to update your current content, consider how much of it you'd want on the .mobi & how often you'd be updating that, and factor that into your decision if you're optimizing the .com or going with a .mobi. For the sites I work on, the issue of content really pushed me into deciding to optimize the .com.