US: Is a keyword purchaser liable for trademark infringement?

Eric Goldman spotted it first: The first US case over keyword advertising where the purchaser, not the search engine, is sued for trademark infringement, dilution and false advertising (Edina Realty, Inc. v. TheMLSonline.com, 2006 WL 737064 (D. Minn. Mar. 20, 2006)).
In France, we had a dozen cases over keywords advertisement on search engine. In some of them, only the search engine was sued, in other both the search tool and the advertiser, and in other cases only the keyword purchaser (below, a slide taken from one of my recent presentations on this topic).