Bukkit

10/29/11 Version 1.0
     - NoJumpBuild is up for download.

It is possible to jump build if the player is sneaking, holds down jump, and continuously tries to place a block.  If the player clicks fast enough, a block will eventually get placed.  However, they will be penalized for each attempt since they will be unable to get the failed-placed blocks back.

NoJumpBuild also disables placing non-block-like items, such as torches, beneath you while jumping.  This will be fixed in the next version.