Out in stores today, I believe it was available on the PS3 Store last night for those who didn't want the box.. I picked my copy up on the way home from work tonight..
"Much more than a demo, Prologue is a fan's taste of perfection.
April 15, 2008 - Polyphony Digital has long been the king of simulation racing. Its Gran Turismo franchise broke onto the scene a decade ago on the original PlayStation and revolutionized what a videogame racing title could be. The second game pushed the envelope, while the third and fourth hit the PlayStation 2 and pushed the genre to unparalleled heights. Some 47 million shipped copies later, Gran Turismo is making its first official "full" release on the PlayStation 3 with Gran Turismo 5 Prologue."
http://ps3.ign.com/articles/866/866698p1.html