We use 'update' mode for our projects, because the checked out tree has local
configuration settings added to it. We also have symlinks to various places.

While trying to figure out why subversion had stopped updating, I did a
$ svn switch --relocate http://... http://user@...

This fixed the problem, but when I rebuilt, Hudson said:

"Checking out a fresh workspace because the workspace is not svn+ssh:..."

It then deleted the checkout, along with all our settings AND the contents of
every directory pointed to by a symlink!

This is probably related to:

https://hudson.dev.java.net/issues/show_bug.cgi?id=585