Report of Inactive Blogs

I need a way to do a report of all the blogs on the site and show which ones have never been updated, or not in a certain amount of days. Is there a plugin like this?

It would be great to be able to email those users based on a certain period as well to encourage them to come back, or warn them of blog deletion, etc.

Please post ideas, including how this might be done so I can develop it myself if needed.

Thanks!