There's not a lot of difference to me between these cars exploding with a nuclear explosion-esque mushroom cloud and cars in other games (and let's face it, movies) that explode when you shoot the gas tank. Both are equally implausible, but few people really bat an eye when you can blow up a car in Crysis or Mafia by blowing up the gas tank, because we've basically been indoctrinated with movie-science that tells us that cars explode when you look at them the wrong way.
To me, the mushroom cloud nuclear explosions is just the Fallout version of basically every stupid action movie or video game you've seen this stuff happen in before. To go into how implausible it is to use nuclear reactors or how microfusion cells wouldn't really explode that way is really giving credibility to the notion that todays' cars really can explode by shooting their gas tank, which is just as insultingly stupid and implausible as what you're seeing in Fallout (or at least, not that far off).
The bigger issue to me, is that the orgies of nuclear explosions going off in this game, be it from the Fatman or these cars or Megaton's nuke or god knows where else you'll see it happening (and let's face it, you probably will), is going to diminish the desired effect from them.
I have little doubt that the first time you see one of these cars explode, or the first time you shoot the Fatman at a supermutant, it will be cool, even if it's only cool to us in the same way that action movie special effects are cool. But once you see it two or three times, and possibly because we're seeing so many screenshots (and eventually we'll see movies, possibly even demos of Fallout 3 with this same effect) we might even be numb to it before we play the game, and at that point it'll just about completely lose its effectiveness.
The sad thing is, normal explosions would actually retain their effect far longer than these mushroom clouds would. Normal explosions at least feel chaotic (even if they're not). But these mushroom clouds seem far too controlled and orderly, which means that once you've seen one or two, you've basically seen them all.
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