If you have downloaded the Gran Turismo 5 Prologue Demo, you will now see that the GT TV icon is active. You will see a little NEW tag on it, letting you know there is content available.
The first three available videos are in standard definition. They are from Best Motoring, Autobacs Super GT, and D1 Grand Prix. Here is a quick video showing the screens and the download process courtesy of kenvsryu555: