On the Grammar Girl website, I explain that the reason most publications capitalize “Internet” is that it’s the name of one specific thing or place, making it a proper noun.
Yesterday I was reading the New Scientist (May 2, 2009) and came across this incongruity: They have an article titled “Is there only one Internet?” (page 32), and their answer is “Probably–for now.” Yet their publication style is to lowercase “internet.” Go figure.

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Our publications’ house style is also to lower-case ‘internet’. I think the logic is that, even though there is only one internet now, there might be more than one in the future so it doesn’t pass the ‘one-of-a-kind’ proper noun test.
Incidentally, the magazines I work for are owned by the same company that owns New Scientist. We don’t share a house style with NS, though.