I suppose so

To get straight to the point. Yes I think it will change the landscape especially in the area of mass appeal for the high definition consoles. Yes it will sell more then Halo 3 due to mass appeal. Yes it will effect Wii sales perhaps not in the area of volume, but perhaps in the area of demographics. The high definition consoles pulling more hardcore or core gamers from the Wii market. Effectively accentuating the divisions between the consoles.

Beyond that I think the people thinking this game will move more PS3s then 360s is basically fan based bias. I gather we learned nothing from last years PS3 sales. Price matters and more then likely the 360 will maintain a large edge in that particular area. People only being drawn in by this game will be far more likely to side with the 360s lower price point, larger library, stronger exclusivity, and the obvious extra content available on the platform. I expect as much as a 2 to 1 margin in sales. Perhaps even greater if Microsoft delivers on another price cut before, or to coincide with the games launch.

The only pertinent question I think is whether Sony and Microsoft have a plan to capitalize on this game, and generate incredibly strong momentum for their brands. To me the answer would be a steady stream of highly prized titles at the very least one or perhaps two a month to generate the real positive buzz they both need. Right now not knowing their plans I am not entirely sure either could manage it.

 Wait, what? 

Why doesn't wii sports count?  I'm pretty sure it's moved/moving a lot of systems.  Also, it's had a pretty massive cultural impact.  Add to this that it is a video game and I don't see why it shouldn't count.  People like to conveniently exclude Wii Sports from any sales discussion when it ruins their point.  

I doubt it will have as much of an impact as Halo 3. It won't get nearly the marketing push and, anecdotally, I don't know anyone buying a console for GTA. Everyone I know eagerly anticipating the game has a console, and everyone else that I know who is interested in the game can wait. With Halo 3, on the other hand was different, I knew a couple of people who ended up buying a 360 for Halo 3. And even with the Halo 3 launch, the 360 didn't do much better in 2007 than in 2006.

There are a lot of GTA clones that do GTA better these days. For example, Godfather was better than the GTAs available so far, in my opinion.

Either way, there is no "market turning point" coming up. A market turning point, when the market has as much momentum as it does, is typically very easy to see coming. You'll see big signs of it. This is a big market, and it changes (relatively) slowly.

GTA 4 will get a lot of its sales from long legs. As people pick up the consoles, a large percentage of them will also be interested in GTA 4.

It will be huge, but for whom?

IMO, it rides on a few things since GTA is popular with the semi-casual crowd who may not have jumped into the 7th gen yet.

- Who drops the price? By how much? What kind of price points will we be looking at? 

- How well will MS advertise the DLC and when will that DLC hit the store?

- Who throws Rockstar the money and gets their tagline at the end of the GTA IV commercials? This is actually a two-front fight in both NA and Europe.

Whoever wins two or more of those will get the bigger impact. MS has the advantage of DLC and price but up to this point, they have been pretty stingy about pricing. Also, if Sony is within $50 (or Euros) of the 360 Pro when GTA hits, that's a win for Sony IMO. 

I find it funny that some of you are bringing up attach rates as proof of anything.

The larger an install base gets, the more diluted it becomes. A 15% attach rate on 120m+ consoles is HUGE. Gran Turismo is the only other franchise that can even come close to those numbers.

In three or four years, remind me to show you how low the SMG, SSBB, Mario Kart, or Wii Fit attach numbers are because the Wii userbase will be so damned large, even though all of those games will sell extremely impressive amounts.

I guess most people just won't care about those titles and they won't be system sellers. 

This isn't a prediction, but I do wonder whether Grand Theft Auto could see (potentially) dramatically lower sales in this generation ...

Grand Theft Auto sold very well in the previous generation in part because it was unique in the market and it received a lot of (negative) press due to the "Mature" content; much like Mortal Kombat many people bought the game (in part) because it had such a 'bad' reputation.

Grand Theft Auto is no longer that unique being that (pretty much) every major publisher has copied the series, and it will (probably) no longer get as much negative press because it isn't a new story anymore. When you add to this that it is being released on systems which are more expensive than the PS2 was I wonder if there will be a dramatic drop in sales.

I think there will be a pretty dramatic drop but I think overall, the game will still come close to 10m sold between the two consoles.

I agree, I wasn't suggesting that it would flop but that there are a lot of things that could lead to lower sales.

 To the dev attach rate does not matter cause they are still selling tons. By the end of the PS2 era there was such a wide range of consumers that only a small number will neccessarily like a certain game. With the 360 in particular, the majority of the games are directed at one demographic, even though there is an ever growing range of games. This means that games of that genre will have quite a high attach rate compared to what you might expect

Back to the arguement, GTA will undoubted sell well, but not to its previous levels. As rocketpig said, they are semi-casual, and semi-casual pricing is what the PS360 are at now, although I expect a price drop from both of them before launch, even if it just a small one