Just saw this today - thought I'd post on the forum as it is a rather large security hole. Apparently an authentication bug has been discovered in Vanilla that potentially allows users to log in as any player (i.e. admin, etc.) They tracked the issue down and put out an update today (9/4) for Bukkit to address the issue.
http://www.reddit.com/r/admincraft/comments/1llt2h/craftbukkit_fix_for_authentication_exploit/
http://www.planetminecraft.com/blog/psa-server-exploit-discovered-update-immediately/
Topic: Security Bug found with Bukkit/Vanilla - Server update required

