Minecraft – How to trigger Command Blocks when a player walks in a certain area

This depends on how complex the system would be and what you want to do with it besides put it on the server. I'll leave you with a pro/con comparison between the two.

Command Block

Pros:

• Easier to create
• Anyone can use the map, not just you
• Don't have to be updated when major changes to the game are done

Cons:

• Redstone circuitry is laggy
• Circuitry can take up lots of space
• Still complex syntax, even though it is easier

Bukkit Plugin

Pros:

• Map is limited to the server, so it can't be stolen using a mod
• Less laggy
• Takes up no space

Cons:

• Can't distribute the map without it being limited to Bukkit servers
• Hard to code
• If you can't code, developers are expensive

If I understood well, you want the /spreadplayers command, which teleports players to random places inside a specified area in a command:

/spreadplayers <x> <z> <spreadDistance> <maxRange> <respectTeams> <player> 

x, z: The middle of the area you want to teleport players (press F3 to see coordinates)

spreadDistacnce: The minimum distance between teleported players

maxRange: The maximum teleport distance from the x and z coordinates

respectTeams: True or false. When true, targets on the same team will be teleported to the same location.

player: The selector of what you want to teleport, eg. @p

Example:

/spreadplayers 0 0 1 10 false @a

This will teleport all players to random locations, but at least 1 block away, from x = -10 and z = -10 to x = 10 and z = 10.

Example 2 (requested in the comments):

/spreadplayers 100 100 0 5 false @r[x=0,y=60,z=0,r=10]

This command will teleport a random player that was less then 10 blocks away from XYZ 0, 60, 0 to a random location within 5 blocks of X, Z 100, 100.

For more information about the /spreadplayers command, visit the Minecraft Wiki or the Minecraft Forum