It's a neat idea, but...
First, it is already possible to explore relationships among concepts in interesting ways by just browsing the site. I'm sure that the vast majority of contributors are like me -- I start reading about one thing for fun, and end up an hour later reading about stuff that is completely unrelated, just by following links which implicitly describe relationships.
Those existing relationships include causality, influence, proximity, temporality, and much more. Why single out "influence" as a privileged kind of relationship?
Second, your example in your original post was of finding out if there was a causal relationship between the Cold War and 9/11. Wqell, I think this example actually points out the artificiality and arbitrariness of how this would end up being used if it existed as a separate feature.
Nearly everything influences everything else, if you want to be philosophical about it. Introducing a feature which tempts people to link "Marie Antoinette" with "Michael Jordan", well, I don't think it makes sense.
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What *would* be cool, and might just be a different implementation of exactly what you have in mind, would be a tool to find all the (reasonably short) click-paths between any two concepts. I mean, now that I selected the article titles randomly, I actually wonder how many clicks it takes to get from Marie Antionette to Michael Jordan. And what's intervening?
Basically, this doesn't need a special type of "influence map", but rather just an analysis of existing links.
--Jimbo