Everybody keeps telling me don't put knives in dishwashers. And if I ask them why, nobodys seems to have an answer or an argument to undermine his/her advice. So I thougt I'll shoot the question to stackexchange:
Do knives get dull in dishwashers?
Yes, knives get dull in the dishwasher, through rubbing against other items.
When I bought a set of nice Wüsthof-Trident kitchen knives, I heard this rumour too. I also heard that cutting onto a plate can dull them. As this was about 25 years ago, in the days before I had heard of the web, let alone a StackExchange, I wrote to my knives' manufacturer, Wüsthof, asking for advice.
They kindly sent me a brochure which has this section (my emphasis):
Caring for Fine Knives
Source: Wüsthof brochure, vintage ~1994. (I still have it, and can scan it in if required.)
I avoided putting them in the dishwasher, until I had a partner who refused to treat them with such respect. (To quote Wüsthof, it was an "outrage"!) For domestic harmony reasons, I got over it.
This article notes that the environment in a dishwasher would cause metals of different electronegativity to react (working principle of a battery), and that the force of the water may knock knifes against other objects. I.e. it proposes at least two specific risks. However, for both mechanisms, the effect depends on other objects. A knife in a dishwasher by its own would be safe.