This is hardly my area of expertise, and not something I've kept up with - hence why I asked - but a few minutes of searching turned up mentions of this happening, along with steps taken to try to mitigate it in the years since I've paid attention to this.At a basic level, unshielded and untwisted wires are essentially big antennas, and modulated signals going through them are going to be broadcast on some level. It's a question of how much and where on the spectrum, and the individual home wiring is going to affect this.

In no particular order:

• Wikipedia article on communication over powerlines (which includes talk about internet over utility lines, which are separate from this and a bigger concern regarding EM noise)
• Article from The Telegraph describing possible prosecution of UK users whose networks cause interference
• Wikipedia article on the HomePlug family of standards, including notes on how the tech has developed to mitigate the problem, but cannot mitigate it entirely.
• An article with some interviews on the subject - covers the subject in text, and the first audio clip has relevant bits start around 14 minutes.
• An article from The Register lamenting lack of remedy for issues cauesd
• An article summarizing the technology, aimed at home users

I don't know if any of the above counts as "authoritative" in your eyes, but there's plenty of affirmation from a variety of sources. I doubt the Telegraph would be reporting on risks of prosecution from things that don't exist. And of course devices sold legally meet regulatory requirements, but that doesn't mean that the problem doesn't exist - just that the problem is "small enough" under ideal conditions, and from the articles I read above, conditions do vary quite widely.