I have a local install. I installed wordpress in a subdirectory folder and using the root as my Blog url. This method is from "Giving WordPress It's Own Directory."
These errors appear in the apache error.log after clicking Plugins.
[Thu Aug 27 14:26:35 2009] [error] [client 127.0.0.1] File does not exist: /Users/username/Sites/www/wp-content, referer: http://www.username.com/wordpress/wp-admin/plugins.php
Changing the Blog Url back to the same directory and not using the WordPress in its own directory method, no longer produces the error.