Wait. Who is playing Prologue? GT5 is finally out!
Someone is playing this old demo for some reason, and I'm kind of glad he decided to pop it in. Mathieu H., on Twitter, has found a secret menu that opens up images of tracks like Nurburgring Nordschleife, Circuit de la Sarthe, and Tsukuba -- tracks that weren't supposed to be in this version. Here's how you do it:
The above command opens a menu that lets you fudge around with the newly unlocked stuff.
Joystiq says that there's also a secret options mode that lets you pull up different cars and options. I think they're right in saying that this is a demo kiosk mode. And before you try, these codes do not work in the full version of Gran Turismo 5.