I ran into a problem today with the way we've been doing things at work. Our repository is checked out in two places: a live server, and a devel server. The devel server is updated to HEAD, the live is not. The live has been spot updated to update things like css changes and such that need to go out immediately. This caused a problem when we needed to roll out some major changes to the live site and could not just 'svn up' because some of these changes require database modifications as well that need tested. Also, we needed an easy way to roll back to what is running live now in case we ran into problems. The solution was found here: svnbook.red-bean.com. This gives you a way to create a 'tag' of your working copy (in my case, what is running live) which provides you an easy rollback process should you need it.
Thanks to kpreid from #svn for this link.