Let's say I want to have several authors for my WP blog.
I want the travel expert to be able to post ONLY to the "travel" category and the cooking expert to be able to post ONLY to the "cooking" category etc.
You might look at Wordpress MU. I've not tried it out, but it looks as if you can set up a different blog for each user. You can set it up at the subdomain or directory level. Each can have it's own layout I think. Worth a look.