has been talked about for years, and numerous trials have been carried out. One stumbling block - as far as the UK is concerned - was that street-light posts were acting as radio emitters and disrupting air-traffic control centres, as well as creating problems for the special radio network used by Police and emergency services. This resulted in early trials being abandoned, and at present nobody is offering a service in this country.
IBM and Google have both invested pretty heavily in BPL (Broadband over Power Line) technology for the american market, and I think it's only a matter of time before we see a commercially viable technology. The power companies are particularly interested for obvious reasons - one of which is that it would allow them to interrogate electrical installations over their supply network - they would be able to detect faults this way, and read meters rapidly and economically.