This exact same thing has happened to me during my first playthrough (no mods installed), however it only occurred later in the game. Now I started another run and right off the bat, the terrain at the path leading up from Goodsprings to the cemetery doesn't load.

http://www.imagebanana.com/view/uwn89q69/ScreenShot0.png
http://www.imagebanana.com/view/k0d7hr7j/ScreenShot1.png
http://www.imagebanana.com/view/ttbj9nmd/ScreenShot2.png
http://www.imagebanana.com/view/ttbj9nmd/ScreenShot2.png

I tried entering the nearest building, saving, exiting and restarting the game, then loading the save and going back there, to no avail. If I take a step into the 'incomplete' terrain, I fall through the floor and plunge to my death beneath the map.

Reinstall and recheck cache it's bugged for you.