Ive been perusing the trailers and dev. commentaries, and from what I can tell, its still quite a bit of wasteland, you are just seeing the settlements because thats what looks exciting in a trailer. I mean, who wants to look at sand for 3-4 minutes of trailer? (minus the people who do it for hours in game. like me.

)
Also, if memory serves, one of the trailers had a screen cap with the map in it. It looks pretty massive, so I am pretty certain that the wasteland can still be explored at your leisure, with settlements being relatively small and out of the way. (Besides Vegas of course.) Finnally, that question was asked in one of the E3 Demos, and the dev guy laughed and said that the wasteland was just as big, and could still be explored by the players.
TL;DR
Yes, you can still roam about plinking zombies and scrounging bottlecaps. There is an achievement for finding 150 locations if that helps any.