I saw this on TechBuzz and thought it was really cool. WP-Wap is not a plugin per se, more like an addition to Wordpress, but it creates a completely WAP compatible version of your blog. Now I know not many people are going to be using a phone to read your blog, but what if there's that one person who's a little weird in the head that wants to do that. You can't exclude that person.

Thankfully, this plugin is just as easy to install as some of its siblings (slightly different, but still easy). Then, it does the rest of the work for you - making the WAP pages automatically.