It seems like Google considers the refferals a relative success and they plan to go beyond their products and add new advertisers:

Many of you already use referrals to direct users to your favorite Google products. Now, with our new referrals beta, you can select products and services from our base of AdWords advertisers. This is good news for those of you who have wanted to use referrals in the past, but couldn’t find a product to match your site’s content. With the referrals beta, you can search for products that match up directly with your site’s content. And you can customize your referral units to match the look and feel of your site, making it easier than ever to find a referral ad that fits in seamlessly with your site’s design and content. (source)

If you are interested you can apply to be a beta tester for this program