tedster - 7:13 am on Aug 24, 2010 (gmt 0)
We began a discussion about this last Wednesday (Aug 18) when Senior Member the_nerd reported the first signs of the change - see Google Now Shows Many Results from One Single Subdomain [webmasterworld.com].
A key observation is that this kind of treatment is quite query specific. In other words, Google's metrics must indicate that the user intention for the query is essentially navigational.
What isn't clear to me so far is the example that netmeg spotted - the search [microformat example] which gives the mega result treatment to microformats.org. Important note that both words must be singular. Make either one of the plural and the SERP reverts to "normal".
Very peculiar since the featured domain IS plural - but it can only be triggered by a singular version!