phpBB 2.0.11, and possibly other versions, with remote avatars and avatar uploading enabled, allows local users to read arbitrary files by providing both a local and remote location for an avatar, then modifying the "Upload Avatar from a URL:" field to reference the target file.
http://www.phpbb.com/support/documents.php?mode=changelog
http://www.kb.cert.org/vuls/id/774686
http://www.idefense.com/application/poi/display?id=204&type=vulnerabilities