I wouldn't say lighthearted. From what I hear the feeling of despair and depression is going to be stronger, not through the scenery but otherwise. There was some talk about it few months ago, and IIRC it was compared to the grittiness of Fallout 2 (which - when barring the humor - was much more depressing in it's atmosphere than Fallout 3).
If they can create that, and leave the over the top humor for lesser attention, it will be good. Though even a bit lesser would suffice.
Aye Fallout 2 had a very depressing atmosphere. Even New Reno, the most colourful settlement, had that dirty, uneasy, not friendly feel about it. What did help were the numerous prosttutes and their pimps and the dozens of drug sellers and drug addicts wandering aimlessly, saying nonsense. The Den was a pretty horrible place to wander into. The children helped a LOT there, as they were poor bastards working for those drug addicts and criminals. They looked dirty, had clearly no education from how they talked and made me look in pity about them, despite playing a pretty evil character.
What will probably help in New Vegas is how grim gambling can be. Add in drug Addicts, prosttutes and gang members and you can have a pretty grim, dark atmosphere for a city with so many lively colors. That should be depressing enough.

Fallout 3 got that atmosphere through its visuals, but not enough with the characters that lived there. Children and citizens were too "clean", everyone spoke well except the raiders and gang members are non existent. Prostitution is also almost non existent, with no brothels. Everyone seemed to be having a good time except some of them. Mr. Lopez in Rivet City, despite not having alot of dialogue, is one of my favourite NPCs because he's the representation of despair and hopelesness in the Wasteland.
Hopefully, New Vegas' soundtrack will be closer to the originals, with less "http://www.youtube.com/watch?v=tuYwAB9p8Lk"-type songs and more "http://www.youtube.com/watch?v=It4GRG7JdFI&p=93027A5CEC1D2949&playnext=1&index=17" types of music.
Edit: Just found this in the comments section of New Reno's song. Very cool comment by a user who played Fallout 2:
This music = I can smell liquor, gunpowder and death. I just arrived to New Reno. The torn-out buildings are filled with fliers, post-war ads and graffiti. The cracked tarmac road hurts my feet. A man named Jules comes to me and offers drugs. Junkies, dealers, lowlifes,? garbage and broken bottles everywhere. I can hear loud moaning at the near-by [censored]house. Also numerous gunshots in the alleys. I check that my revolver is loaded...just in case.Exactly how I feel when I listen to the song. The soundtrack always helps a lot in setting the mood of the areas you visit.