User blog comment:Dopp/Technical Update: May 31, 2011/@comment-3104099-20110601031733/@comment-1393830-20110601182452

That would be really hard as it would require a complete overhaul to the way an undo works. Undo works by restoring a page to a previous version but if someone else has changed the page since the edit you want to undo, you would be overwriting that other change.