Nautilus began to manifest annoying sluggishness, which was the immediate result of it hogging up the CPU anywhere between 40 and 100 percent any time I did pretty much anything involving files or directories in the GUI. After covering some solution attempts which turned out to be dead ends, I created an extra user account for testing purposes and found that it was not susceptible to this issue.
Finally, I found a bug report on Launchpad with the “high cpu load” issue precisely described, and luckily with a fix included. I stripped my desktop directory (“Työpöytä” in my locale) from the variables defined in ~/.config/user-dirs.dirs (so that they referred to $HOME), replaced one symbolic link there with a genuine path, and then ran xdg-user-dirs-update && xdg-user-dirs-gtk-update before logging out and back in again. And now Nautilus is as nimble as ever.
Out of curiosity, I tried to reproduce the issue with my test user account: I inserted a symbolic link into one of the variables, as I suspected that was what triggered the sluggishness, but it turned out not to do so. So the exact cause still eludes me.
The updated Nautilus packages including a fix for bug #454764, which I suspected for being related (since at some point Nautilus kept dumping “signal 11”), have been released into Lenny. When I still thought this was the core issue, I was worried they wouldn’t make it.