In all seriousness though he has commented on "Pervert's Guide to Ideology" that the whole appeal of post-apocalyptic fiction is that when you see civilization stopped and dead is when you truly feel the weight of history (he says that this is an idea he borrowed from Walter Benjamin, but I tried to find the book where he talked about that to no avail).
If I were to haphazard a guess as to what Zizek would say about Fallout if he knew it even existed I think he would probably notice the undergoing aesop about heroism in that being too fierce about chasing your version of the greater good ends badly: On every game there is aways some big antagonist that thinks he has got it all figured out how to make the world a better place (The Master, The President, Henry Eden, Mr. House) and then goes for it without restraint, in Fallout 1 this extends not least to the protagonist who is exiled from his Vault because he saved it. In a way it's like the game is passing the message that the world has no place for ardent and proactive self-righteousness and that self-control and tolerance is more important, that would be why the factions that seek to make a positive change into the world and still endure are the ones which tame their passions (Followers of the Apocalypse, (western) Brotherhood of Steel, NCR).
In fact it is like the zealotry of the antagonists is so great that they often didn't really even think their own plans very throughly and can in almost every case be simply "talked out of it" by the protagonist once they point a flaw on their thinking.
All put into contest - that is, the death of the world at the hands of nuclear warfare - this moral message becomes an criticism to the unrestrained and self-righteousness aggressiveness promoted by ideologies such as jingoism. In the specific case of the United States - the culture of which the game dances around with - this in reality served and still serves as ideological justification for atrocious aggressions the world over all in the name of the "greater good" (democracy or bushian "peace").
Then he could perhaps notice a turning point in the way Fallout 3 depicted ardent heroism, that is, in the way it glorifies it. Fallout 3 is depicting the same actions of unquestioning ardent heroism and yet it twists the moral message the other way around by making the deaths of the heroes (James and Vault Dweller or Sarah) messianic in nature. Having this in mind it is not, perhaps, coincidental the usage in Fallout 3 of the Bible and american national symbols under pious and nationalistic airs (which does not happen under even remotely similar lights under any of the other games). Fallout 3 went the other way around and instead of exploring skepticism it embraced the ideology of unquestioning self-righteousness.