To those dismissing the announcement because Final Fantasy games suck, it's not the game that's a big deal. Regardless of our unpopular tastes, this is going to have a major impact on the current PS3 vs 360 competition.
I disagree that this singular title is going to make a huge impact on PS3 and 360 sales, as Final Fantasy ceased being a system seller years ago (in the U.S.). Looking at the gradual decline in each successive title's sales figures, and more importantly the steeper decline of U.S. sales, gives a pretty good indication that few people would've ran out and bought a PS3 just for Final Fantasy.
It's not going to single handedly change the market, but it was the last big hold out that Sony had other than MGS4, which will likely also make it to 360 within a year. It was a key piece of a puzzle that Sony needs to put together to remain a serious contender. If FFXIII ever
does get released it will still be one of the biggest PS3 titles that's been announced so far.
The 360 would be dead if it weren't for Halo 3 and Gears of War (and without GoW2 they might be in trouble). There are other important 360 games, but they likely wouldn't have landed on the system if the big games hadn't kept it going in the first place. The PS3 needs at least one big game to identify itself with. MGS4 is the closest they've come, but it's surrounded by a cloud of 360 port rumors that Konami doesn't try hard to deny.
The first Final Fantasy of each new generation seems to have a big impact. FFIX and FFXII suffered from coming out it the later half of their console's active lifespans and were kind of 'more of the same' tech-wise. FFXIII certainly has a lot of 'buzz' going for it and early on it was supposed to be one of the games that showed what the PS3 could really do (and that the 360 couldn't). Not only has Sony's claim that the PS3 is a full generational leap beyond the 360 been proven bunk, the general public can now see that they're more or less equal in overall graphics. So now it's all about games.
The real measure of how much of a deal this is, would be the reaction to the announcement. Which I think it's safe to say is pretty big.
Most games will be cross platform. Only 1st party games won't. So I guess it's just a matter of whose 1st party games you like more. Halo? f*ck dat.
This is exactly my point. Halo 3 and Gears of War are two of the biggest games of this generation. MGS4 should be the first game of this profile that's exclusive for the PS3.