Just to throw in my five cents, I never even realized that Fallout takes place in an alternative timeline until I
read about in a article on the game.
(On a sidenote, I only read that article because my half-year old Super-Notebook has crashed and now I cannot play my newest Chrismas present.
)
And I actually played both Fallout 1 and 2 quite excessively, exploring a lot and actually reading almost all of the in-game information that I could find in pre-war bases, etc.. It appears to me now that this whole "alternate timeline"-thing seems a bit more prevalent (or rather: blunt) in FO3, with new dates for space program and all, whereas in the original FO games, I could completely overlook it. Of course I noticed the 50s look of cars, pre-war art, etc., and the strange design of computers, etc., but I did not really put too much thought into it - after all, the theme was consistent, and I hardly felt the need to look upon the details of technology in a world where radiation creates two-headed cows and giant scorpions. For what it's worth, I usually accept far more anachronistic combinations of technologies from different epoches in the run-of-the-mill fantasy/ medieval game than a "waterchip" looking like somewhat 120 years older.
I just assumed that there had been a cultural backlash to the 1950s in the 2070s, a 'Renaissance' of sort, and in my eyes that seems still more plausible than the cultural standstill canon seems to imply.
Anyway, to sum it up:
1. IMHO the "alternate timeline" thing is of little importance to the setting of FO.
2. I would even assume that it was deliberately toned down in both FO and FO2, because the designers wanted the player to explore the world of FO, rather than trying to explore what had happened in the alternate timeline.