In the Quake 3 engine, and others, the forward slash is used to denote that you want a cvar command (if you typed it without the slash, then it would be a "say" command, and broadcast to everyone in the game).
/pwned may be said on purpose rather than commanded to show emphasize the action of the pwnage without using many exclaimation marks, 1's, and oneoneones.
Kenthar: *While N00b is typing, knifes him in the back of the head* /pwned
N00b: OMG Typekiller! *leaves*
Admin: /pwned n00b
Server: N00b has been PWNED!
N00b: Abusive admin!