Robert_Charlton - 6:40 am on Aug 18, 2011 (gmt 0)
I think it's a brilliant feature. These are for navigational searches, and they're much more useful than classic sitelinks. While they do dominate the page, the two columns allow Google to squeeze a lot of navigation into a reasonably small space. There's room for several other listings above the fold, and after even a 12-pack there are still nine other standard results on the page (if you count Universal results, sometimes more than nine).
Popularity of the page seems to be an important factor in whether it's included in the list. Popularity also appears to determine the display order. Note that a page in the list apparently needn't be linked directly from home. For a local weekly newspaper site, eg, I'm seeing an important article from 2009, which made a very big splash, returned as one of the twelve. In this case, there's no homepage link to be found.
On the sitelinks for WebmasterWorld, OTH, one of the sitelinks is for the current top Highlighted Post. Right now, on 8/17/2011, this link is: New "Show More Results", going to the New "Show More Results" Infinite Scroll thread in Google SEO News. It will be interesting to note how often it changes and what replaces it.
I'm wondering what kinds of sites these are where Privacy would be a primary concern. In the WebmasterWorld sitelinks, small-font links (albeit from the top of the page) that made the list include "Recent posts" and "Search".
I'm guessing that measures of user engagement will allow Google to calibrate the new sitelink algo even more. I also feel that these sitelinks go hand in hand with Panda. They should provide some hints about how Google is trying to improve engagement on its own pages.