What do you mean probably the PS3 soon? I've been playing the game for almost two weeks now.
If I'm not mistaken, the overworld in Oblivion would be streaming off the disc/hard drive install, meaning it's constantly loading new sections as you travel. In fact, the game frequently gives a loading area message whenever I'm travelling very quickly through the overworld. It even chugs occasionally as the game tries to keep up with me. In the grand scheme of things though, as large as the world of Oblivion seems, it wouldn't be that large when travelling over it in an airship FF style. You also just can't load a world that size, with that much detail with the speed necessary for something like airship travel. You'll notice most overworlds tend to be pretty small and simplistic, specifically so they do load quickly and easily. As long as Square continues to go the route of travelling through areas as though you are actually travelling through the world, rather than the abstract representation of FF's past, you won't get control over airship travel. Certainly not this generation at least.