Last year, people started to notice that Google began showing "subtopic" or "subcategory" links below their listing, if they were the first in a Google search. Now, Google's finally confirmed the change as a permanent feature and given it a name: sitelinks.
Social Patterns dug into this a bit more. He determines they are "likely determined by traffic patterns." His findings in short:
Google snippet links do not return links outside of the home domain.
Google snippet link text can be determined from an image's alt text.
Google snippet links can be subdomains of the home domain.
Google snippet links are not determined by PageRank.
Google snippet links are displayed for the top result for a "brand" search or "domain" search. (For example, "zappos" and "zappos shoes")