I added a slide summarizing the discussion on why top-k results
should simultaneously satisfy two constraints (be as close to the query
as possible while being as far from each other as possible).
As I said one way to increase result diversity is to get results in terms of their
proximity to the query, and then cluster them and give cluster representatives.
Vivisimo.com is such a seach engine.
Here is an example:
(you can try other queries..)