Might not affect 1.4 but I have no way of knowing due to the lack of a Bukkit build for 1.4

https://bukkit.atlassian.net/browse/BUKKIT-2283

Sent with player abilities packet, it appears there are 1 or 2 lines of missing code which cause this functionality to not work.

setting player flight speed works though. This would be useful to freeze players and other stuff