404 for wp-login.php after 2.6.3 upgrade

I just updated to 2.6.3 from 2.6 using SVN by doing a switch (svn switch http://svn.automattic.com/wordpress/tags/2.6.3) and then updating the database (http://mysite/wp-admin/update.php). Now the frontend of the site works fine but I can’t login. Going to wp-admin redirects me to wp-login.php (http://mysite/wp-login.php?redirect_to=http%3A%2F%2Fmysite%2Fwp-admin%2F) and I then get a 404 error. And yes, wp-login.php is still there.

Any ideas why this is happening? I suspect something with URL rewriting, though the fact that the front end stuff is still working makes me wonder. I have the WP install set up to use a static frontpage and then put blog posts under http://mysite/blog/. The fact that this is still works just adds to the mystery. The .htaccess file looks unremarkable, and just sends everything to index.php.
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