While I have several misgivings about fallout 3, I don't believe it's worth anyone's time to harp on whether or not the original two games or any sequel adhere totally to the 1950's motif.
About the only people you'd really see adhere to that( and that's debatable) are the people in isolated places like the Enclave, or as yet, unopened fully-functioning vaults. Places where the populations, and the spaces they live in, resemble living time-capsules of a bygone era.
But even that is going to be tough to maintain with every succeeding generation that is born. Unless the people of the vaults deliberately cling to those superficialities like a security blanket/anchor in order to keep themselves from going insane. The 50's style/culture then becomes more a matter of indoctrination- with more chance of it becoming a caricature of itself as time progresses because the stimuli that formed it in the first place just aren't there anymore.
Speech, clothing, hair, morals, and even technology(assuming humanity begins inventing anew instead of simply scavenging/reusing) are going to start evolving/devolving as soon as you leave the vaults, enclaves- whatever. Just like it did in America and the rest of the world as the 50's passed onto the 60's-70's 80's etc...
The Brotherhood of Steel is an example. Some chapters might act like your typical hardcore military unit- very clipped and by the numbers. Armor, equipment, and organisation held to the standard that the original units who fought in them maintained.
Others might buy into the ideal of knighthood so totally that they change their mannerisms, speech, hairstyles, dwellings, morals, and might adorn or customize the look of their armour and weaponry to reflect that ideal- in effect creating the look of some throwback to the time of King Arthur and his Knights of the Round Table. (painted/enameled power armour suits, chasing or fluting or filigree added to armour or weapons. Melee weapons like hammers/swords/maces with modern touches. Riot shields/armour sporting heraldry, reinforced concrete outposts/fortresses with river stone facing to evoke images of an ancient castles/bastions if kept in this context- the robot horses- yuck I know- start sounding not so bad)
Let's also not forget the wild west/frontier justice flavour that crept into the first two games. Small main street(the only street) boom towns- ranchers, bandits, lawmen. Killian came across as a post apocalyptic Wyatt Earp. Whereas Gizmo was giving off a heavy "Al Capone" vibe. They felt totally incongruous, but somehow they worked. The same went for the different families you encountered in New Reno in Fallout 2.
The main thing Bethsoft(or anyone) needs to do when making these sorts of changes is to provide proper(plausible) back story explaining how they came about, and then make sure that that they remain consistent within the context of the world from that point forward. If they can do that, without making the reader/player feel like they've been duped, then it stops mattering so much if Optimus Prime is walking around the wasteland or not.
Now, that said, if they just drop a new element in there for the hell of it- without offering some decent setup via writing/dialogue- then there's trouble. And, that having been said, I still don't have much/any hope for Fallout 3.
