All of San Andreas?
I believe so. The trailer showed a lot of country side. The trailer did not show any other cities thou, but neither did the original GTA:SA trailer. I believe Rockstar is just holding out details, or those parts are not complete yet to show in a perfect trailer form as Rockstar loves to do.
Also Rockstar didn't have planes in GTA:4 as they thought the map was too small and since it was only one city their was no point. Maybe this time planes have somewhere to fly.