Looks like a Demo of GT5 is coming out this month!
"Starting December 17, fans will be able to download a new demo of Gran Turismo 5 through PlayStation Network. The Gran Turismo 5 Time Trial Challenge features the Nissan 370Z, both tuned and non-tuned, as well as a one track. "
Full article below:
http://ps3.ign.com/articles/105/1051607p1.html