As an Automattic deletion ;) of inactive blogs would be a little overkill (users need a chance). An email (configurable with how long it is inactive) to the blog admin stating that the blog will be deactivated in 7 days if a post is not made. Thereafter it could show up on the Site Admin with a nice Check Single/All (and view blog) and Delete (this one should be manual). That way you can keep the blog for another 30 days or so (or forever).. also a message (in very booring format) to replace the users main page stating that if you are the admin you need to login and post to reactivate.
Seconds?
blips
