In an attempt to update Drupal to it's latest version (6.5) I decided to test it on my local setup. Lo and behold, I got an 'Access denied'. After testing out several possible causes, I googled around.
The answer: a few days ago I setup an SMB share on my home folder, with access for only my user account, no access for 'everyone'. That resulted in a permissions change on my home folder, even after removing the SMB share.
sudo chmod o+rx [home folder]