We've started to test a new Bukkit build that should kill serverside lighting updates. what this means is, 99% of the lag will be gone. You will notice dark areas if you do mass sniping. This is an unfortunate side effect for now. Flying by those areas will update it to the server. Any FPS lag you get it because of clientside lighting updates. Thank Mojang for that.
Edit: it's off for now. found some issues that need to be fixed.
Edit: Back up. Enjoy testing.