Eirik,
bit slow on the reply, watched some football earlier and now wish I hadn't bothered...

Anyway, checked a couple of racks which hadn't ever been apart, from what I could see.

Picture of the caps here, showing the (factory?) drilling in each for a split pin.
< http://www.flickr.com/photos/30132857@N06/4498470590/ >

I tested how much each cap would turn inwards before removing them and it was about 1/4 to 1/2 turn, so it's beginning to look as though that mark on the pinion is the problem.

If the car had slid and hit a kerb hard, could that have chipped a tooth?

ken