Spot-on analysis from John Battelle on the reasonableness of Google using search created context to serve up image-based ads as well as text ads across AdSense sites:

“In other words, it won’t be long before Google combines the contextual relevance of AdSense text links with more brand-driven, rich media ad units. And that means they start becoming a major ad serving service in the vein of Doubleclick and its kin.”