I can now provide more citations. From the Wikipedia article (http://en.wikipedia.org/wiki/Internet_capitalization_conventions):
“In the Internet standards community, which includes the IETF, usage historically differentiated between the common noun, with a lower case first letter, and the proper noun, with an upper case first letter. That is, “the Internet” (capital I) referred to the Internet, while “an internet” (lowercase i) referred to any internetwork for connecting multiple networks together — including the use of Internet technologies for this purpose inside private networks. The distinction is evident in a large number of the Request for Comments documents from the early 1980s, when the transition from the ARPANET to the Internet was in progress.
“Another example is IBM’s TCP/IP Tutorial and Technical Overview (ISBN 0-7384-2165-0) from 1989, which stated that:
The words internetwork and internet is [sic] simply a contraction of the phrase interconnected network. However, when written with a capital “I”, the Internet refers to the worldwide set of interconnected networks. Hence, the Internet is an internet, but the reverse does not apply. The Internet is sometimes called the connected Internet.
“The Internet-internet distinction fell out of common use after the Internet Protocol Suite was widely deployed in commercial networks in the 1990s.”
Clearly, I’m showing my age.