Well I’m sure Polyphony Digital won’t like to commit to a solid release date, since they're famous for extending development to producethe perfect product. But a Sony executive has revealed that we can expect the game to arrive before the next Christmas. 
I personally expect the title to fall into 2010, but we can always hope. Sony’s Chief Executive in Southern Europe, James Armstrong, outed the news in the Spanish newspaper El Financiero: 

“For next year, the chief executive of Sony Computer Entertainment for southern Europe said that they are developing a new game thriller, in which the player may be designing their own history, and the new version of Gran Turismo, which will arrive at Christmas.” 

Usually Gran Turismo comes out earlier in Japan, so it could see a summer 2009 release there. What’s more exciting is that Polyphony Digital have openly admitted to making updates available for Gran Turismo 5 Prologue before the release of Gran Turismo 5. 

These updates are said to include a full damage system and even weather effects. If Gran Turismo 5 is set for a late 2009 release, Prologue should get damage modelling much earlier in the year. That is, if PD get over the issues some car makers have with the implementation of car damage in such a realistic game. 
http://www.gamezine.co.uk/news/game-types/driving-and-racing/gran-turismo-5-late-2009-=-prologue-damage-early-2009--$1252251.htm

this seems possible

GT5 will be out in 2009, the maker of GT kazumari watever said he was just joking about a 2010 release date, and said it won't take that long. but your right, they are mainly concentrating on updates for GT5 prologue than the actual game. goes to show they really care about making the best game possible

i love the prologue, but i don't want to wear it out, so i stop playing it after i beated it and i just wait for GT5. Hope GT5 lives B3yond its hype

@FPSrules

I'm playing GT5: Prologue right now, the only gripe I have is that I wish damage was on and I had a DS3 controller to feel the feedback.

But other than that I am looking forward to the actual release of GT5. Plus the updates that they will be giving Prologue.

Is it me or is the whole we dont want virtual damage on our 100,000 dollar car completely retarded ???

@PAINTRAINWooWoo

I think its retarded, but really I can see their point not wanting really good looking cars get damaged. But they our trying to make this game as real as possible. It's not real if the damage is not in the game right?

I totally agree, but I can see where they are coming from, as they want everyone to always think their car is the best. If it is bashed to pieces next to a competitors car in perfect shape, that wouldn't look good.

Anyways, I'm interested in getting this game, but was holding out for the full version. If they add weather and damage to GT5:P, what will be the big difference between the 2 versions?

@nordlead

The big difference is the game wouldn't be confined to a demo, it will actually be a full-fledge release of the actual game.

More cars, more tracks and etc. They spent a good wealth of development on this game so the majority of everything we see in GT5: Prologue will be in GT5, but will be significantly huge and bigger with options.

When that update arrives for GT5:P arrives it will be amazing! Sales are a solid 20k per week at least and that should only help

But how will an update improve sales? lol