I’ve created a plugin called Word Statistics for WordPress 1.2 2.0 that displays readability statistics on the post editing screen whenever a post is edited or saved (and you continue editing). The plugin also provides similar functionality to the MovableType plugin MTWordStats. I’ve never used MT or tried the MTWordStats plugin so I can’t really say how the two compare.

Update 12/30/2005: A new version has just been released that is compatible with WordPress 2.0. The older version is still available and works with versions prior to 2.0.