What algorithms does the mains stuff use? Are they published? If not, the security is likely to be rubbish. The electricity meter isn't likely to block it either. Years ago I used to install incoms for old peoples' homes that worked over the mains.
Yesterday I drove for 10 minutes with netstumbler on a PDA, picked up 20 access points of which 10 were open.
The rest were plain "64-bit" WEP, and if those networks are used for something that routinely carries a lot of data (e.g. driving a printer) then airsnort would recover the key in an hour or so.
This was just with an old PDA that's sitting in a hands free kit running a GPS road nav package - not a lot else works on this PDA! The range was poor; one would have to park right in front of the house. But anyone doing this for a "job" would have a directional aerial and would be easily doing it from several hundred metres away.
Either, this needs to be done properly, or one should just lay some twisted pair cabling around the house.