I wrote another plugin for WordPress that limits the categories that are displayed on teh edit post page for users who do not have a high enough user level.
What does this mean?
If you are participating in some kind of guest-blogging activity, like Free For All Friday, then you can turn on this plugin and your guests will be able to post only to the categories you specify.
You have to edit the plugin file to specify the categories that you want to limit guests to, set the breaking point for what user level is high enough to see all categories, and specify the option of whether you want to allow subcategories or not.
This plugin isn't comprehensive. Don't get me wrong - it works, but a determined user can work around it. It just makes it obvious what you want them to do with their posts. I still have to do a little work on it to make it perfect, but it's a good start so far.
Download from the hacks section, as usual.