Gmail has been displaying RSS Feeds and sponsored links in a box called 'Web Clips' above the main frame. Users can customize the RSS feeds displayed but are forced to display ads when Google thinks it might be useful to you. Until recently, Gmail has been displaying ads in this format:
Today, I saw this format in my account which similar to the link units in AdSense:
On click, you will be brought to a page full of ads and some related pages. Gmail has been implementing link units several months ago next to the email messages labeled 'More about ...'. If these keywords displayed on link units are high relevant to the content in emails, users will more likely click on them then the ads which has a title and content. Now that Gmail is testing link units in our web clips, it might be a sign that Gmail isn't as profitable as you think it is and Google shall be finding more ways to monetize it.