In the New Architect article, "Joint Venture", Adaptive Path's Jeffrey Veen discusses how to co-brand services successfully on the web.

Well-executed partnerships can make your site's offerings both more complete and more competitive. Just remember that new content or services will only attract users if they are complementary to your current offerings and are consistent with your existing interface. And without an unrelenting focus on your users fueling your development, you risk having a confusing site and alienating your core audience.