Facebook is testing several new Facebook Ad formats - recently rolling out surveys and 'become a friend' buttons inside the ad units. These are additional ways to brands and marketers to engage with their audience on Facebook - either directly (ie by collecting polling data) or in an ongoing basis (by converting them into Facebook Fans). Remember, not long ago, almost all of the ads on Facebook took users offsite... but Facebook's marked growth and the increased usage of Facebook Pages has made on-Facebook marketing more effective and attractive.
Facebook is now testing another advertising format: direct response units. InsideFacebook has screenshots of an integration with Chick-Fil-A where users can "get a free chicken biscuit" by signing up for a coupon via Facebook. That action is then viral as users are encouraged to share the event with their friends (like the Starbucks Free Ice Cream giveaway) and it shows up in their feed. Over time, it makes sense that Facebook will allow bundling of actions: sign up for a free sample and 'become a fan'.
These ad units are win-wins for consumers and brands. When Starbucks broke the 3,000,000 fan mark - they asked their fans what they wanted to see from Starbucks on Facebook; the resounding answer was coupons, offers and new product news.
Images from InsideFacebook's Facebook Testing New Direct Response Home Page Ad Unit