Plugin Idea!

Jack, there's a neat plugin on bukkit called HerobrineAI, I thought you should try:

http://dev.bukkit.org/server-mods/herobrine-ai/

Edit: OI. It really is an AI. You need to go to the config.yml and put him in the world you want him in. Like, the resource world (maybe). Also, there are commands you can do for trolling. I'll tell you what he can do and what the commands are:

HerobrineAI Plugin adds:
Herobrine (AI)
Graveyard World

commands:

/hb-ai help (/hb help): Shows the current list of commands avail.
/hb position: Gives real In-Game location where Herobrine is.
/hb attack (playername): Disorients, and Attacks the specified player.
/hb haunt (playername): Haunts the specified player. (if not set, he'll go about haunting anyone alone or with one friend)
/hb graveyard (playername): Sends the specified player to Herobrine's graveyard (No one can access it or once put there, cannot move for 10-20 seconds).
/hb bury (playername): Buries the specified player in a 4x4 rectangle (won't be killed.). Can either mine themselves out, or ask someone to go to their grave and help them out.
/hb pyramid (playername): builds a pyramid nearby the specified player.
/hb cancel: Cancel Herobrine's actions on the player who he currently targeted.

That's most of the commands I know.