In Japan, the demo for Final Fantasy XIII is available right now, to those who purchase the Blu-Ray edition of Final Fantasy VII: Advent Children. But you don't have to book a flight to the Far East if you just want to check it out. YouTube has you covered.
The very, very end of the demo is not included in the vids below, and reportedly reveals an important non-spoiler. It contains a card confirming that Final Fantasy XIII is coming to Japan in Winter 2009. In The Land of the Rising Sun, you'll only be able to play FF XIII on your PS3, but here in the States, the game will be out on the 360 and PS3, supposedly in 2010.
The demo videos as embedded below the cut. It's three parts long, and obviously the spoilers are MASSIVE. Click at your own risk.