Google is facing yet another challenge to its advertising system, AdWords. The system has been challenged before by brand holders such as GEICO and American Blind & Wallpaper Factory (ABWF), but the latest is from one of the world's most famous companies, American Airlines.

The beef is simple in outline, but complex in detail. Google sells ads beside your internet searches which are related to whatever you typed into its search box. The problem is what happens when you type American Airlines or AA into that box.

AA wants only its ads to appear. Google sells the right to display ads next to that term to whoever buys the space. That is the conflict.

A US court ruled in one previous case that using trademarks as triggers was OK, but using those trademarks in the ads themselves was probably a no-no. That, said the judge, caused consumer confusion. We were just about to get blessed clarification on whose fault that would be when Google settled in secret with the other half of that case, GEICO.

Another case involving ABWF has had an initial hearing but will proceed to a full jury trial.

AA is clearly not happy with the outcome of the GEICO case, and has been pretty stiff in its pre-trial verbiage about Google's policies. It says its trademark policy is "manifestly deficient" and it wants a jury trial and punitive damages.

Google has not yet fought this battle with as large, as experienced and as deep-pocketed an opponent as AA. It has another possible adversary, though: Google Europe.

While Google allows US and Canadian trademarks to be bought as keywords, its rest-of-the-world policy allows no such thing. In Europe, it blocks the use of trademarks even behind the scenes as keywords.

"Google appears to have the ability to structure and configure its programming to stop this misuse of the Amercian Airlines marks because it has already implemented procedures with respect to European internet users," says the AA case.

So Google will have to defend in court a policy that its European arm does not practice against a battle-hardened business veteran from a cut-throat competitive and highly litigious industry. If Google hasn't grown up yet, it will soon.