According to a report from CVG, Valve's dropped titles on Steam from Ubisoft that's gotten the firm's ill-fated DRM software in its games, including Assassin's Creed II and Silent Hunter V.
Originally, both games were available here and here respectively, but they are no longer where they should be. A senior source told the site Valve doesn't want to lose face over "its reputation for customer service, and these games have angered its user-base."
Whatever happens, this ain't going to go away for a while yet. We've emailed Ubisoft for comment.