I discovered this new method of turning your blog which covers several main topics into a collection of smaller niche sites. You are effectively creating isolate categories which each target a seperate keyword phrase. This may have already been done, but it's something I believe can be profitable. Read more at > Optimizing your blog for a bunch of keywords Comments appreciated : )
It's no great revelation. Just look at BBC.co.uk for an example of a site that has many different minisites enclosed within the same domain.