How do I I prevent passwords from being saved to the keychain?

I have managed to find an SVN-specific answer.

It seems that the SVN client itself uses whatever password caching functionality is available on the underlying platform, but this can be controlled with a configuration file.

Specifically, configuration files for SVN can be found on a Mac or *nix box in the "invisible" directory ~/.subversion (i.e., ".subversion" within your home directory). In the current release, to unconditionally suppress all password caching, go to %/.subversion, and edit the "servers" file in the editor of your choice. Find the [global] section, and within it, the commented-out line,

# store-passwords = no

Remove the pound-sign and the leading space (if you leave the leading space in, the svn client will fail -- I learned this the hard way). The line should now read

store-passwords = no

And that will unconditionally suppress password caching in the Eclipse client.