Search Engine Roundtable and Darrin Ward blogged about site links in Google Adwords. As you can see, Staples listed on Google sponsored and organic results with sitelinks.

Sitelinks appear not all the time and only for big brands and companies. Most likely Google is testing new feature with handpicked participants. If you type Walmart.com or BestBuy.com for example, it appears as regular link in sponsored results.