Eigenname.

From the article you posted:

Internet is traditionally written with a capital first letter, as it is a proper noun. The Internet Society, the Internet Engineering Task Force, the Internet Corporation for Assigned Names and Numbers, the World Wide Web Consortium, and several other Internet-related organizations use this convention in their publications.

Many newspapers, newswires, periodicals, and technical journals capitalize the term (Internet). Examples include The New York Times, the Associated Press, Time, The Times of India, Hindustan Times, and Communications of the ACM.

Others assert that the first letter should be in lower case (internet), and that the specific article “the” is sufficient to distinguish “the internet” from other internets. A significant number of publications use this form, including The Economist, the Canadian Broadcasting Corporation, the Financial Times, The Guardian, The Times, and The Sydney Morning Herald. As of 2005, many publications using internet appear to be located outside of North America—although one U.S. news source, Wired News, has adopted the lower-case spelling.

Und in der Diskussion zum Artikel steht noch mehr dazu: http://en.wikipedia.org/wiki/Talk:Internet#Ca...

http://www.worldwidewords.org/topicalwords/tw... has a long description of the word's history.

Not sure if I agree with the reasoning, though:
"And just as we don’t capitalise the words for media such as television, radio, mail, telephone, or newspaper, why should we capitalise Internet? "
...to which I'd answer: because there is only one Internet, but everybody has a telephone, or even several. It's like the difference between the Moon and a moon.

To be honest, I'd not expected to have been able to find the answer to my question in the Wikipedia article - for me it was just a convenient example to cite.

However, the arguments are interesting and all in all, a good example of how language evolves.

Thanks for your replies.