It is difficult to know what signals Google uses in its Sitelinks algorithm, but a close inspection of the search result pages for queries which return an extended Sitelinks - and a thorough look at the Google Webmaster Guidelines - do provide us an opportunity to make some reasonable assumptions. There are in fact several signals that websites should consider addressing if the allure of extended Sitelinks is too much to ignore.
Based on the current research, it seems that whether Google includes Sitelinks is heavily dependent upon the behavior of users. Since search results page listings include tracking info, Google can/could easily determine which pages or categories of your site are most frequently visited. Thanks to this click stream data, Google is able to improve its search results and give users direct access to the sections of your Web property which are of the greatest interest or importance. Google generates sitelinks automatically but with a little strategic thinking, it is possible (somewhat) to influence extended sitelinks. If you don't already know the most popular sections of your website, it's time to dig into your website analytics.
Perhaps most noteworthy within the Webmaster Guidelines is the recommendation to make a site with a clear hierarchy and text links. If Google cannot understand what sites or individual sub-pages are about, it is unlikely that extended Sitelinks will appear. To increase the likelihood of extended Sitelinks appearing for your site on important queries, consider rethinking navigation.
For example, consider refining the navigation to include but a handful of the most important links. Fewer navigation choices will help you funnel users to specific destinations, and giving Google enough click stream data to make some choices on your site's behalf. Take a look at Apple.com's navigation, and then at the extended sitelinks Google has returned on the search results pages, and tell me that there is no correlation between navigation links and the sitelinks that returned.
Google needs not look very far for perhaps the strongest signal of a page's worthiness to be included in Sitelinks. SEOs should focus much of their energy on the quality of page titles (keeping them brief but accurate) and doing the same for the actual file names of pages as both are strong signals that specific pages are of importance. Take a look at the Ameritrade example below. See how the titles are short in length? That is certainly one good indicator. But dig deeper into the "Rates & Fees" Sitelink and you'll see that the page itself is named ratesfees.html. If the aim is to obtain extended Sitelinks, considering a site's naming conventions (titles and pages) is a vitally important task.
Another opportunity is to ensure that the internal links within your site and external links pointing to your site from third-party websites use an appropriate keyword anchor text. The aim should be to identify which pages are most representative of the site in question, and to set about acquiring a mixture of links with the keyword or key phrase anchor text you want to have your site appear with extended Sitelinks.
Extended Sitelinks are obviously a new addition to the search results pages of Google, so expect development to continue over the next several months and years. As it stands today, the sites of well known (established) brands are those which are receiving the extended Sitelinks. Take the necessary steps today to acquire extended sitelinks and you will likely be pleased at the virtual real estate you secure on the search results pages.