John Gruber, on the casing of “Internet”:
The Internet is a lot like the Earth. It’s everywhere. It is our universe, in a sense, from the human-scale perspective. But it’s a unique and distinct thing, thus deserving to be treated as a proper noun. It’s almost disrespectful to lowercase it, and the Internet is one of the great achievements in the history of mankind.
It’s also grammatically wrong. As my Networks lecturer would say, “the Internet” — the thing hosting the Web that we use everyday — is distinct from “an internet” — a collection of interconnected networks. The Internet might be an example of an internet, probably the prime example, but there’s no law in computer science stating that there must only be one. Anyone is welcomed to make their own internet. No-one will because there are massive network effects associated with network protocols, not to mention the amount of money already invested in the TCP+IP stack. Why do you think it’s so hard getting people to move to IPv6?
That said, the style guide of this site has no hard requirements over the casing of “Internet”, mainly because it’s easier not to press the shift key. 😀