Required fields in post edit

I may be going to use WordPress for a group that supports the local independent bookstore. It will therefore be a site with multiple contributors.

I think that most, if not all the presentation of the site is going to be based on categories – events, reviews, members’ news etc.

The problem is, therefore, that contributors are going to write a post and, in their hurry to see themselves in print, publish without attributing a category, which means that, although published, their article will not be seen.

Is there a way (hack or plugin) that makes the selection of a category obligatory ?

Another way of saying it would be can one make categories and perhaps tags required fields with some message that tells the contributor to enter data ?