A simple way to get mulitple blog functionality is to add a new table called simply "blogs" and a new row to every table in the database called "blog_id."
The blogs table would have information such as blog name, blog description, etc, etc...
The blog_id in the tables would have the id of the corresponding blog. Then each blog could be called via a variable in index.php (i.e. index.php?b=2). With mod rewrite it would look like each blog has its own directory.
This functionality is very similar to categories but i would say it would be more like super categories as each blog would have their own seperate categories, users, links, etc.
It would also be good if you can assign some users/categories/links to multiple blogs instead of having them twice in the database (easily implemented with many to many relationships in the database).
Any other thoughts/opinions/flames??