Powerline Ethernet and ring mains

I don't like wireless in my house with kids around. Powerline Ethernet sounds like a very good alternative, but I have a couple of questions I hope Register Hardware readers can answer...

Do the units communicate with each other even when they'reon separate ring mains, ie. a ring main for upstairs and ring main for downstairs?

And how secure are they? Can anyone in the same block of flats or next door plug one in and connect to you?