Google's Webmaster Help Center has added some information about those extended descriptions that offer extra links to deeper sections of the website. These results are sometimes seen at the #1 postion of the SERP, and there has been some ongoing question and conjecture about how they are chosen and when they are displayed.
...We only show Sitelinks for results when we think they'll be useful to the user. If the structure of your site doesn't allow our algorithms to find good Sitelinks, or we don't think that the Sitelinks for your site are relevant for the user's query, we won't show them.
At the moment, Sitelinks are completely automated. We're always working to improve our Sitelinks algorithms, and we may incorporate webmaster input in the future...
Msg#: 3076513 posted 3:31 am on Sep 10, 2006 (gmt 0)
Some of the algo also seems to use site architecture, so a clear architecture helps. Some experiments I ran last year suggest that links close to the top of the source code stand better chance of showing up as SiteLinks.