Bukkit

Have you updated? You should update WorldEdit to 5.4.5 or newer and (if you're running MC 1.4) CraftBukkit to 1.4.2-R0.2 or newer.

Changes in this version:

• Added mobs.block-above-ground-slimes to block slimes spawning naturally above ground.
• Added security.deop-everyone-on-join to de-op all players when they join.
• Added security.block-in-game-op-command to prevent usage of /op from in-game.
• Fixed a NullPointerException in PlayerInteract involving water potions and potion blocking.
• Fixed an issue with snowballs being blocked in regions.
• Fixed a regression that appeared in 5.6.2 that caused paintings and item frames to no longer be properly protected.
• Changed dragon eggs to be checked like it is a broken block in regions.
• Moved developer's WGBukkit class to package com.sk89q.worldguard.bukkit.

Quote from the WorldGuard team:

• Stay up on WorldGuard news! Subscribe to sk89q's Twitter
• Chat with the developers! Get to know us! Get help! Get on IRC!

Changes in 5.6.3:

• Added helper WGBukkit class to get references to WorldGuard from other plugins.
• Added physics.vine-like-rope-ladders to make ladders work like vines. As long as the top ladder block is present, ladder blocks will hold even if they have no block behind them, like a rope ladder.
• Fixed NullPointerException in PlayerInteractEvent caused by potion blocking.
• Fixed mobs.block-{item-frame,painting}-destroy not working.

Changes from 5.6.1 and 5.6.2:

• Added a mycelium-spread region flag.
• Added a dynamics.disable-mycelium-spread configuration setting.
• Added new gameplay.block-potions option. For example:

  gameplay:
      block-potions: [invisibility, speed]
  

• Fixed a bug in the entity explosion event handler that caused a NullPointerException.

Changes from 5.6:

• Added official support for Minecraft 1.4. Support for 1.3.2 (and below) is still provided in this release.
• Added support for new and existing explosion types.
• Added support for blocking withers with the following settings:

  mobs:
          block-wither-explosions: false
          block-wither-block-damage: false
          block-wither-skull-explosions: false
          block-wither-skull-block-damage: true
  

• Added support for blocking item frames (and their use) in protected regions.
• Added item frame destruction region flags (if you wish to disable item frame destruction completely).
• (Re-)added support for priority R-tree region indexes. This means quicker region lookup, and better support for servers with thousands of regions.
• Improved /region info output for -g group flags.
• Allowed removing of -g group flags with /region flag.
• Allowed /region flag to set group and value at once.
• Deleting a region flag now also deletes group flag.
• Fixed detection of attack by projectiles to be more accurate.
• Fixed support for saving non-string flags in the MySQL region support.
• Fixed spurious comma in "Flags:" /region info output.
• Fixed players who are not in god mode not receiving damage in PVP areas from enderpearls.
• Players will now be told that they are in a no-PvP area if they are in one and attempting to attack someone, or that the player they are attacking is in a no-PvP area, if that is the case instead.
• Fixed an issue where region.getId() was lowercase'd each pass of a for loop in MySQLDatabase.updatePoly2dPoints().
• Fixed mob damage from projectiles not being properly blocked in regions whereapplicable.
• Changed default region wand to leather (#334).

• WorldEdit should be updated to version 5.4.5 or newer.
• CraftBukkit should be updated to version 1.4.2-R0.2 or newer (for MC 1.4 users only).
• If you want to block Withers spawning, use "WITHERBOSS" and not "WITHER".
• For other issues, see our issue tracker for bugs affecting this version (updated automatically).