I used NuclearW's spawnarea on my own server for a long time until it broke after a bukkit update. There were other options, such as RandomPort, but I didn't like the fact that it told every user "You have been RandomPorted to ..."

So I created PlayerDisperse, my very first plugin. It has one permission node, and one command:

Command: /pd [radius]
Permission Node: playerdisperse.pd

If [radius] isn't specified, it defaults to 4500.
It teleports the command issuer to a random point within a square around the center of the current world (0, 0), within the specified radius.
It won't teleport the player on lava or water, below sea level or inside any material other than air.

How to install:

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Place PlayerDisperse-0.1.1.jar in your craftbukkit plugins/ folder.
Restart your server.
Success

There is no configuration.

Recommended for usage with Shereis' amazing plugin ScriptBlock:
http://dev.bukkit.org/server-mods/scriptblock/

Feel free to check it out in use on my personal server:
IP: mc.example6.org
PORT: 25565