Speaking as a dyed-in-the-wool Fallout fanboy with more hours logged for F1, F2, & F3 combined then I've probably spent eating food, the idea of getting to explore a massive and fairly fully developed (hell, I'm not expecting them to do every bloody restroom and water closet, but give me some awesome casinos to go wild in, I'll be happy) New Vegas is VERY appealing to me. Personally, though, I'm going to go out on a limb here and suggest we're not going to be JUST in New Vegas the whole game. It'll be the center of our world, yes, but it's just too damn tempting to show what's out there beyond the untouched/refurbished New Vegas they're currently flaunting in our faces.
Remember, folks, it's Fallout. Do you REALLY think they'll be able to resist tossing it at LEAST Area 51? Or a decent simulacrum?
Bah, whatever. It could be little more than a graphical update of New Reno and I'd still end up wasting twenty hours just making my character work his way up the boxing circuit and only then get around to taking out rival gangster family members.