It's been done... and it's cheap...

ZyXEL PL100 Powerline Ethernet (about US$60-80)

* High Speed 85Mbps Powerline Solution
* Uses existing power cables and plugs
* Easy to install and manage (Centralised management utility)
* Up to 300m - 3 times the distance of standard Ethernet over copper wires
* 56 Bit encryption for added privacy
* 16 PL-100's per private group
* 4 Groups per network

By the time you get a design, and the parts, and make a board, and do the program and factor-in all your time, it'll cost you ten times what walking down to the shops and buying one would cost. Then you can use all that extra time in taking it apart and learning how they did it...