Actually, since we're all talking about the leak, I've looked it up on the net and it seems that it comes from our all time favourite Gamespot(tm of course). They don't say how it happened, but one of their leading reviewer, Greg something (the guy who did the video review for Oblivion, Thief III, etc.).

Anyhow, it's not a beta, but a full version sent out to Gamespot for reviewing purposes. How it appeared as a Torrent, nobody knowns or wants anybody to know (most probably somebody just making a copy for themselves and their friends, who in return made a copy for the WWW).

Wikipedia confirms this leak on their FF12 page:

http://en.wikipedia.org/wiki/Final_fantasy_12#Leak

On October 1st, 30 days before its official American release, a full English review copy was leaked and uploaded to many P2P networks and Bittorrent trackers.