I quite like the Wordpress Multisite model which deploys a separate set of tables for each blog in a single MySQL database. Then you can add on the HyperDB plug

I quite like the Wordpress Multisite model which deploys a separate set of tables for each blog in a single MySQL database. Then you can add on the HyperDB plugin which lets you create rules to split the sets of tables into different databases. This gives a lot of flexibility.