So one of our interns just asked whether we capitalize "Internet."
A good question, since, IMO, it is a political decision (although grounded in grammar).
There was a time when we defined "the Internet" as a proper noun. The Internet was the singular "network of networks" using the TCP/IP protocol.
Increasingly, folks no longer capitalize "internet." In part because people generally don't think of "the Internet" as a proper noun. Frequently it's because they confuse specific applications with "the Internet" (for example, "the World Wide Web" (which also used to be a proper noun) or IM (not a proper noun, but usually capitalized as an acronym).
But there is also a political component. Phone and cable companies love the idea that we have lots of internets (just like Ted Stevens says). In particular, they like the idea that there is a "public" internet separate from the "private" internet the phone and cable companies build. Although what these companies really mean by "private internet" is the last mile access connection purchased by the subscriber when the subscriber buys "internet access," plus whatever bells and whistles the access provider tacks on.
For further reference, feel free to read my professional blog entry, What the #$@! Is The "Public" Internet? at http://www.wetmachine.com/item/417
I told our intern to capitalize "the Internet." And I note that whil "interns" are common, "the Internet" is not.