Here is a plugin which I had thought about a long time ago, and has now come to fruition: category access.
I’ve released a new version of the Category Access WordPress 2 plugin. This plugin allows the blog maintainer to restrict access to posts belonging to particular categories on a user-by-user basis.
This would allow you to easily implement a member system on a blog, or premium content for registered users. A very nice idea. Check out the plugin page at Coppit.org.