I agree with Richard. Bad idea.
If you have a chat to any of your radio techs/engineersthey'l probably curl up and die with anguish at the abuse of the radio spectrum.
What you get with mains wiring (and transmission line) systems is a huge increase in the noise level across the bottom end of the radio spectrum. Mains wiring needs a carrier frequency to be applied to it which carries the data signal. The data signal has to be quite broadband (can be 11MHz if a crappy encoding algorithm is used!) in order for you get the promised bandwidth for your PC network. The reason CAT5 & 6 systems work so well is that they use properly terminated transmission lines which are effectively impedance matched so that there is minimum loss of signal bewteen the sender and receiver. If you have a bad match, that results in what are known as standing waves which can lead to all kinds of dire consequences one of which is radiated signal from the transmission line ( e.g. house mains wiring).
Another drawback is that mains wiring isn't screened (normally) and that means that it can pick up signals as well. That in turn could mean that your data gets corrupted and so the network has to resend it, which reduces the overall efficiency of the thing. You'll be lucky to see 5Mb/s in a real home situation.
I would suggest that if you do go down the mains wiring route that the security issue wouldn't be a big one anyway because Id hope that the designers/manufacturers of the units would have implemented some sort of security protocol which you can configure to make sure only your network can use the data.
Please please please don't encourage the abuse of the radio spectrum with this "technology".
Whichever way you go, good luck.
ST