I used to have an intercom system from Radio Shack which used the house mains to communicate between stations. (It's still somewhere on the top floor - I came across it the other day). Living in a three storey house, we used it to get my son out of bed and ready for school some time before lunch!
On one occasion we were treated to one of the neighbours doing exactly the same for their children (never did find out who, but they must have lived at least fifty yards away as there weren't any other kids closer), so I guess that using the same system for linking computers wouldn't be exactly secure - rather like hanging around trying to find hot spots on other peoples' wireless networks to get a free connection. There would appear to be nothing to prevent data being transmitted back through to the mains for other people to pick up, or someone linking into your machines.
GG