[quote name='sixteenvolt420']Well, i consider anything that isn't the full game, a demo, even though this is longer than most demos.[/quote]
I see your point. This game is 40%-50% of the full game, and can be viewed as an attempt by Polyphony Digital to cash in twice on the very same game. However, they've said that it'll be at least a year until Gran Turismo 5 comes out.
Also, I think they'll continue to allow those of us who've bought Prologue to continue playing online with those who wait to buy the full version of the game (though obviously we won't be able to play all the courses).
Depending on how much more we'll actually get for another $60 investment (and it's safe to say at least 20% or more of the people who buy Prologue will also buy GT 5), I may not buy the full version of the game. I own every version of Gran Turismo, and find myself interested in the games for a good 20 hours or so. Because of that, I may not even want to buy the "full" version of the game.
Another reason I hope the "demo" (as you put it) does extremely well (which it will) is because it'll send a message to the game developers. $35 or $40 is a price we're willing to pay as a consumer. $60 is too much money for a game, regardless of the amount of time that games provides us. Hopefully Sony will put two and two together when Prologue outsells GT 5 3 or 4 to 1.