403 Forbidden error using Subversion
Another instance when this issue will rear its head is if you commit a file twice with the same name but with different capitalizations (e.g., foobar and FooBar). This is only possible, of course, on a windows system and may just be a special case of Todd's answer above. One of our developers accidentally did this and it similarly cost us many debugging hours.
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.
We run into the periodically and it is very frustrating to developers. For some reason reading from the repository seems to be case insensitive but commit cares.
I understand the reason why case matters due to Subversion's roots in unix filing systems that are case sensitive, but I really wish you would get the error on the initial checkout not on the commit!