PS3Fanboy are the bearers of bad news, Square Enix have said "at present the FINAL FANTASY XIII demo will only be available in Japan with the Blu Ray release of Advent Children Complete, but we hope to have further news in the coming months." This means that there are currently 'no plans' for a demo of Final Fantasy XIII outside of Japan.

The demo will be bundled with the Advent Children blu-ray and won't be available by any other (legal) means. However, this doesn't mean there won't be one, it just means they haven't planned one yet. You could always import it anyway.