GT5 Prologue: Demo is out NOW
Only thing is, you need a Japan Playstation Store account. Here's a link to show you how to sign up for it.
http://blogs.ign.com/xheavenxsentx/2006/12/09/39684/
have fun
PS: It's only available from today until November 11th, so don't wait.
If anyone can translate Japanese, that would be great because I don't know how to progress haha. Also, what's the option that's under transmission selection when selecting a car?