I'm not the only one with this problem, so I found clues here linking to here.
Unfortunately, that was outdated information, and fortunately, it is now a bit easier:
Probably first make sure that the keyring package is installed.
apt-get install debian-archive-keyring
Then go to the clazzes.org launchpad site. Follow the instructions there! This involves adding a key for that package using apt-key, and adding the deb files to /etc/apt/sources.list
sudo apt-get update
sudo apt-get install subversion
comment the clazzes.org repositories out of /etc/apt/sources.list and rerun
sudo apt-get update
Messy, but it works. (Only for subclipse 1.4.x!! Not for later/earlier versions)
