Since multiple discs is being talked about...
It just seems odd for a GTA game. The story missions are the only thing that could be split across discs. Think about the different aspects of the game, most of the content is potentially available from the start and will need to be available at any time.
Here's what would need to be on every disc:
The map - All of the models, textures, animations and audio that make the game world, as well as the data that places vehicles, weapons and NPCs in to the world.
The vehicles and weapons - All models, textures, animations, audio, stats and physics/handling.
The NPCs - All people and animal models, textures, animations, audio and AI behaviour.
The Radio - Hours and hours of audio.
That's not even comprehensive, there's more engine stuff like the lighting, there's the weather, the scripts that run what happens when you buy from a store or use a Pay N Spray for example.
The story missions themselves do contain a fair amout of data in audio, mo-cap and scripts. If they were able to split the story across discs it would be a solution assuming the game is too big for a dual layer DVD, but it seems crazy to me that they would have to release a game across multiple discs where 80-90% of the data is the same on each disc.
Maybe someone who has a better idea of how much disc space things take up could chime in. The install size of GTA IV on 360 (as best I can remember) was 6.8 GB so I guess they've got roughly 2 GB extra they can use for V before even worrying about multiple discs.