Wasn't it stated that DVD was a challenge, if not even a problem, in addition to PS3 being a difficult platform (or something like that)? Both platforms had their problems.
However, that problem doesn't exist now. Even though the game's been developed on PC, they must have taken PS3 into account for a long time now. Besides, as it started as an exclusive, I'm sure they've focused on taking advantage of PS3's strengths. You get my point. We've already seen that PS3 isn't that difficult or bad when you do exclusive games, and tech-wise we can largely consider FFXIII as a PS3 exclusive (sort of, I hope you understand what I mean).
However, I'd be more worried about DVD this time. If it was a challenge with GTAIV, now it's a problem. Even if the game's linear, the world isn't. As far as I know, a world map has been confirmed; it basically means that you have access to most of the world at some point of the game. As the world must be largely put on each disc (well, could depend on design), it's even worse. Of course cutscenes and voice acting will probably take most "additional" space; they don't have to be on every disc. Also, voice acting and other sounds ought to be pretty easy to compress without too much loss in quality.
What I'd be worried about are CGI cutscenes. HD video needs much space, and thus I'm unsure if they can fit on DVD. Depends on their amount I suppose, but even a few can take much space. The 360 version might even have to have SD video instead of HD video which is what the PS3 version is probably getting.
We should also remember that the PS3 demo takes up some 5GB. I'd be willing to guess that at least 1GB, possibly even 2GB, are videos, but even then it would leave 3GB-4GB for the demo alone. If we compress that to 50% (can we really even do that?), it'll still be 1.5GB-2GB - more than most demos. If we apply the same to the full version, assuming the PS3 version needs a full dual-layered BD (50GB, that is), the 360 version would still be 25GB. AFAIK there was some copy protection or something on 360 DVDs which drops the available space to about 7GB (pretty hard to imagine why 8.5GB - 7 GB = 1.5 GB is needed for copy protection though, but that's what I've read a few times; last today). If 7GB is available, the game will take about four DVDs. If the whole 8.5GB would be available, it would still need three DVDs.
Yeah, they're probably going to need at least three DVDs even if they use excellent compression methods. And that's really if they use some efficient compression which also means loss in quality.
Then again, I probably lack the knowledge to make very accurate predictions about this so maybe I've gone a bit too far. Still, I'd say that should give a very good general idea about what's going to happen. I guess it'll be four discs max, probably three. Four is pretty bad for MS's image, but it's also getting pretty expensive (and remember, dual-layer DVDs are more expensive than usual single-layer DVDs you usually buy). Thus I'm guessing that three discs is going to be the most likely final number.