Courtesy of Coy Day's favorite constituent, Bill Smith, W5USM, I've learned about a pretty good article describing BPL at
http://www.eei.org/magazine/editorial_content/nonav_stories/2004-03-01-Broadband.htm
published by the Edison Electric Institute. There's a sidebar near the end describing the four parts of a broadband network: the last mile, the ISPs, the middle mile, and the content providers. The "dark fiber" mentioned by Dick is the "middle mile."
 
The article devotes five paragraphs to radio interference. We might quarrel with some of it, but it certainly puts anyone reading the article on notice that radio interference is a real issue.
 
Dave K1ZZ