I was using WordPress via the Subversion 2.0 branch so I had to do:
svn switch http://svn.automattic.com/wordpress/branches/2.1
and then clean up a few spurious merge conflicts.
Interestingly, while writing this post I might have spotted a WordPress 2.1 bug – I had written:
<code>svn switch http://svn.automattic.com/wordpress/branches/2.1</code>
but WordPress kept trying to do some linkification or something that screwed up the above svn url and caused the rendering of the post to end prematurely. I worked around this by hiding the URL from WordPress by writing:
<code>svn switch http</code><code>://svn.automattic.com/wordpress/branches/2.1</code>