It would be nice if they released the GamesCom demo for us to download, or the TGS one with the 458 Italia. I played the Prologue demo from when it was released to Prologue's release, which, if I remember correctly, was a very long time...
Prologue isn't a demo, it doesn't have any of the traits of a demo: The feature set is apparently very different to what GT5's are now, it has over 70 cars and 6 tracks with alternate layouts (name any other demo with that sort of selection), and, most importantly, it cost money. I've never ever paid for a demo in my life, nor have I played one as much as I've played Prologue.
The GamesCom demo, Tokyo R246 with just the WRC Impreza, that's a demo. The Prologue demo with what was it, 8 cars maybe and just Suzuka, that was also a demo. GTHD, with 10 cars and tuned versions of each, two modes (time trial and drift) and just Eiger, that to me was more like a free minigame.
I kind of have some hope they'll release a GT5 demo after there's been some playable ones at shows, and given that in this console generation we can download things and we've already had three downloadable GT releases (a demo, a free minigame and then half, quarter or maybe even 1/8th of a game) over the past years I don't think its entirely unreasonable to believe they'll release something we've already seen shortly before launch. That said, I'm not going to pretend its a life or death situation, nor that I really expect a demo, but it could happen.