I ran into this problem when trying to upgrade several of my WordPress-powered blogs through Subversion. The cause was that I had replaced the default theme directory with a copy of my working theme, due to the ‘reverts to default’ bug which, by the look of things at WP’s support forum as well as my own experience, is still alive and well.
The default theme is part of the svn repository and svn expects to find its own data on the current status of the files, but a copy of my own theme doesn’t have the data, so it complains about the missing or not locked
working copy. The solution is to delete the default theme directory based on my own, then do a svn up to fetch a copy of the original default, then do a copying-over: cp -r my-working-theme/* default/
Now the working copy of the default looks like my own, yet is seen by svn so that a svn sw http://svn.automattic.com/wordpress/tags/2.3.3/ (or whatever the new version is) is possible. Note that editing style.css is required to tell the default apart from the actual working theme. I also like to use the screenshot of the original default to make it stand out when the working theme once again gets replaced by the default.