Different category = different domain

Not possible afaik, if only because one part of the required wordpress options is the domain name, which needs to be different for each unique site. What are you trying to accomplish with this?

An option you might want to look into is multiple blogs on a single install : http://www.google.com/search?q=wordpress+multiple+blogs then you can have a single userbase, with whatever shared tables you would like, but have separate urls