<p>I recently upgraded to Subversion 1.5, and now I cannot commit my code to the repository. I get an error message: "403 Forbidden in response to MKACTIVITY". I know the upgrade worked because my fellow developers are not getting this issue. What's going on?</p>

Answering my own question:
Apparently my SVN URL had the wrong case! A Google search turned up an article (no longer available online) that explained what was going on. My URL was of the form
http://svn.foobar.com/foobar
but the actual repository was called
http://svn.foobar.com/fooBar
.
I use TortoiseSVN, so the fix was to use the Relocate command to correct the path to the repository.
Hopefully this will help someone else.