I don't know about over power lines, but inside homes its a very slow form of networking (1Mb/s unless you use some special hardware last time I checked). I wouldn't think you would need a modem as such, although for my ADSL connection I don't dial-in yet the device I have is still called a modem (well, it's a router with a built-in modem if you want to be picky). Stricly speaking you'd need something to convert the signal from your computer to whatever frequency is sent down the power lines, which is what a modem does by definition (MOdulating DEModulating).