the aim - a mainsite where a user can sign up and create their own blog and select an appropriate category for the blog topic. This should then be searchable from the main site.
similar sounding requests all seem to point to the 'Blog Topics for WPMU' plugin, but after installing this I am still no closer to being able to show a list of all blogs associated with a certain topic
Large sites such as unblog.fr seem to have mastered the blog category management.
Does anyone have pointers on how to achieve this?