When it comes to trademark protection, don't sue over keywords. Eric Goldman, who follows search and related tech cases closely, says "it's often irrational to bring lawsuits over keywords." Not to mention the waste of time and money, too.
Yesterday, one of the better-known cases was settled between American Blind & Wallpaper Factory and Google. American Blind wanted to prevent competitors from buying keywords which they had previously trademarked. If the case had gone to trial, then it would have been over the matter of consumer confusion.
This means there's been no U.S. based test about trademark protection for keywords, at least not yet. Google clearly states its position, in its trademark policy: "Please note that we will not disable keywords in response to a trademark complaint."
If your company owns trademarks, free and clear, then remember they won't be protected when advertisers want to buy them as paid links. However if the trademarks appear in ad copy, you have recourse. Follow the current procedures with Google or other paid search providers, to file complaints for removal.