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.
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.