A related question came up on StackExchange two years ago - they wanted a default category by author. For the programmers out there: http://wordpress.stackexchange.com/questions/45398/automatically-assign-posts-by-author-to-a-specific-category/45403#45403
I used this on a client site as a few authors kept forgetting to select an appropriate category.

A defunct plugin seems to provide similar functionality in a nice package: http://wordpress.org/plugins/wp-adc/