Patrick Mackenzie, of Bingo Card Creator fame, uses what I'll call micro-sites to dominate certain keywords.
For example, search for "halloween bingo cards" and you'll see the first hit is http://www.halloweenbingocards.net which is a micro-site of a few pages of halloween specific content about bingo cards.
If you search for "bone bingo cards", you get a link to his main site with one page of content about bingo cards and the human skull.
He owns the long tail on basically X + bingo cards, but went all out to build a custom site for the halloween bingo card search (note, the domain name almost guarantees this as a first hit in search results).
If you had a bunch of long tail content sites, how would you figure out when it makes sense to pull one out and make it into it's own standalone site at a different url?