1337 is "leet" which was spawned from the quake/team fortress days. it means "elite". pwn3d is the h4x0r way of spelling pwned, which was popularized by CS (Counter-Strike) players who typed "owned" but accidentally wrote "pwned" whenever they made a headshot (a shot to the head... duh) because of their need to type quickly in order to move and not get shot themselves.
refer to this article by Microsoft for a crash course on "h4x0r" a.k.a leetspeak or "13375p34k"
http://tech.msn.com/guides/1064107.armx?GT1=6210