The cash cow keeps getting healthier. CNN.com has agreed to join Google's AdSense advertising program, which has made the search vendor the multi-billion-dollar Internet ad realtor it is.

Neither the financial terms nor the duration of the deal were made public, but a CNN spokesperson confirmed Google is replacing Yahoo as the provider of choice for contextually relevant text, image and video ads next to content on CNN.com. Google will be the exclusive provider of auction-based text advertisements throughout CNN.com.

Such deals are common and often circular. Google's leadership in search rankings may give it an edge in landing the bulk of such contextual ad deals. But it's partly through such deals that Google continues to be the search leader. 

To be sure, the CNN agreement is a coup over not only Yahoo but Microsoft and AOL. All of the search and online portals covet advertising dollars and eyeballs for their content and additional Internet services.

In July, Google renewed a multi-year deal with Washingtonpost.Newsweek Interactive whereby the search provider drives contextually targeted ads and Google Web Search functionality to washingtonpost.com's Web pages.

A year ago this month, Google became the exclusive search and keyword ad sales provider for the properties of News Corp.'s Fox Interactive Media, including MySpace.com. 

But Google's rivals aren't strangers to such deals either.

Earlier this month, Yahoo added Philly.com, online home of the Philadelphia Inquirer and the Philadelphia Daily News as an online ad partner.