They won't, you'll have to swap discs when visiting certain areas of the map.
Not true, don't spread misinformation :nono:
PS3 uses Blu Ray, so everything will be able to fit easily without having to compress anything. For DVD's they'll just use compression techniques, which Todd said they've gotten much better at during this current hardware cycle.
I very much doubt the game would fit a blu-ray disc if it was 100% uncompressed. I think you would be surprised to see how much space uncompressed graphics/audio use.