The Guardian's style guide[1] has it in lowercase, as does the BBC News[2]. Reuters[3] is slightly more nuanced:

  Capitalise as a noun, lower case as an adjective e.g. internet banking, internet cafe.

[1] http://www.theguardian.com/styleguide/i

[2] http://www.bbc.co.uk/academy/collegeofjournalism/how-to/news...

[3] http://handbook.reuters.com/index.php?title=I#Internet