I added a section
http://www.mediawiki.org/wiki/Subversion#Undoing_changes
with some tips how to efficiently use "svn merge" to undo changes, especially how to undo a specific changeset.
In addition, a hint was added how the
"svn merge ... --dry-run"
option can avoid headaches before doing something wrong.
I'm mailing you this so that everyone has a chance to improve or correct the page - if this is needed.
Tom