Image by via FlickrJust in case you're ever tempted to say something stupid like "Ubuntu is unkackable," watch this video, then re-think the statement.
The video shows a local root exploit in the Linux 2.6.30 kernel. Karmic will ship with 2.6.31 so this exact exploit isn't likely to still work. Oh, and even though the vulnerability is local, remember that remote user exploit plus local root exploit equals remote root access. ;)