Where's the beef (again)?

America is a difficult place to understand sometimes. Well, most of the time. An extraordinary degree of importance is attached to TV ads showing during the Super Bowl, and this year a General Motors ad (here, on YouTube: embedding doesn't seem to work)  has caused a furore. Bear with me - it does have a legal aspect.
The commercial plays on the Mayan calendar's prediction that the end of the world will come in 2012. According to the GM ad, surviving the end of the world (which, when you think about it, is a pretty pointless thing to do) depends on driving a Chevrolet Silverado.
Ford took exception, because the guy who didn't make it to the meeting place after the apocolypse drove the Ford competitor, the F-150.  The F-150? Isn't that the Formula One car? Or am I confusing it with the Ferrari pick-up? Either way, Ford seems to have suffered another corporate sense-of-humour failure: according to GM, the ad is an over-the-top spoof with "the devastation and destruction predicted to occur this year by the Mayan calendar [including] giant attack robots, meteors and frogs falling from the sky." GM's Global Chief Marketing Officer Joel Ewanick said:

We stand by our claims in the commercial, that the Silverado is the most dependable, longest-lasting full-size pickup on the road. The ad is a fun way of putting this claim in the context of the apocalypse.

The ad implies that the Silverado is more durable than the F-Series pickups, with Ford countering that there are more of its trucks on the road with at least 250,000 miles on them. Ford's lawyer has written to GM demanding that they "immediately cease and desist from making any unsubstantiated and disparaging claims regarding Ford's pickup trucks."
GM, still in the spirit of the ad, claim "we can wait until the world ends, and if we need to, we will apologize," continuing (probably not believing their good fortune at the additional publicity being generated by their rival):

In the meantime, people who are really worried about the Mayan calendar coming true should buy a Silverado right away.

Now (to pick up the legal theme in this story again), different countries have different approaches to comparative advertising. It almost invariably involves the use of a competitor's trade mark: in the Silverado ad, the F-word is used - and I don't mean Ferrari's parent ... So there is a prima facie trade mark infringement, except that most laws allow you to use your rival's trade mark to indicate its products. In the UK, it would have to be in accordance with honest practices in industrial and commercial matters and not take unfair advantage or be otherwise detrimental - broadly speaking it would have to be fair. You don't have to study American advertising practice for long to spot that they are rather more liberal over there: which makes it even odder that Ford should have reached for its lawyers over something like this - giving GM the oxygen of publicity even though it seems they had no intention of going further.
In a rational world, Ford's remedy would be to take the corresponding advertising slot next year and come up with something as amusing and memorable. If, that is, there is a next year.