I just upgraded from the previous version, maybe this is a bug in WP 3.0.
I receive a 404 error message after activating "wp-admin/edit.php" (to list all posts) and "wp-admin/upload.php" (to list library content).
Other pages do work. When I add a post or new media then there is no problem. This can be a bug in WordPress of in the redirect?? (if there is) I don't know.
Maybe someone can test this.