If you're over in the US you'll know this as BPL or Broadband over Power Line, over here you're more likely to see it some nice little devices you plug in the mains to get your network to somewhere using the mains. So what's wrong with that? No running network wiring or trying to get your wireless network to go the distance?
A brief look at the technology-
By using ODFM modulation, you have a large number of carriers spread over a very wide frequency range, which you then use the mains wiring to carry the data. The idea is for AV applications, computer networking etc.
The US had this also supposedly as a way to get broadband to rural areas over the grid, but that's more or less died off due to technological difficulties and the FCC taking note of the problems. In the UK it's more commonly tagged onto a "BT Vision" installation.
So what's wrong with that?
It's just that mains wiring is designed to carry power and that's all. Over the recent years, the amount of conducted and radiated noise on the mains has been the subject of considerable legislation, so filtering on devices has improved (with a few exceptions - low quality PC PSU's have been of some concern). Along come these devices and we're back towards the days of thyristor chopper motor controls before the filters.
The unbalanced nature of power distribution systems makes them unsuitable for data, the phone system has always been balanced, it ensures better immunity to noise both ways, and has proper termination. Power systems have all sorts tagged on them, heaters, motors and devices with nice input filters. They are also branched, have odd lengths and in no way uniform in their implementation. This turns the wiring into an antenna, it's unavoidable. That is unless we retrofit everybody's home wiring with industrial type screened cable.
The effect is to wipe out everything within a few hundred metres on everything between 3 and 30MHz. Of course it may be said that this spectrum is not really that important, only the hams, and to a minor extent the military and airlines are affected. They are supposed be be "notched" (ie lower emissions) in the ham bands, but this can be less than expected and still kill the 80m band stone dead with wall to wall mush. One service completely killed by this is the lower end of radio astronomy.
There are some newer devices which promise "gigabit" performance that mush up to 300MHz and these are of real concern and will get noticed by the public. FM broadcast, a lot of PMR (business radio), DAB, Air Traffic Control etc are likely to be very adversely affected. This is the point where public safety becomes a concern. It might even wake up the regulators here in the UK (OFCOM) who seem to be only handling complaints on a case by case basis.
We've had a demonstration at my local club RADARS about this. It wiped out reception from the main HF antenna about 80ft away, and the club's underground and covered with several feet of concrete for screening.
So what are the alternatives?
Good old Cat5e works very well, it's not that obtrusive, and it's easy to deal with. It's also quiet radio-wise and reliable. Wifi also works, although some may crave more bandwidth, but it's pretty much universal. Also creeping up are mini optical fibre, which looks like the long term winner as it's much easier to lose down the edge of the carpet and round door frames.
The Yahoo Group UKQRM has much material to look at, and it gets plenty of mention elsewhere.