I've found two chips that seems to offer a complete solution for wiring two arduinos over the domestic electric cables:
ST7538
MAX29X0
They are relatively cheap (10-20 euros).
I'm not an expert, how about interfacing them with Arduino ?
http://www.sparkfun.com/commerce/product_info.php?products_id=8974
seems like that would be a more complete solution
(i hate to post so many sparkfun links but they always seem to have this odd kind of stuff)
I've been keeping an eye on the powerline communication chips, but haven't seen a simple solution. Certainly nothing like the LM1893, which was discontinued long ago. The Maxim solution is trying to get multiple megabits of communication over the power lines, rather than a simple serial protocol. It uses multiple chips, some are BGA. The ST7538 is not too bad, it does have a pretty complicated array of configuration registers and need a decent amount of external analog design. I am really hoping to find a solution that can be rolled out for, say, 5 or 10 bucks per node, making it feasible to automate everything in a house.
The TDA5051 seems interesting, though it's an older chip and may be discontinued soon too.
The sparkfun board is the demonstration that this is possible at "low" cost.
The other chips seems to be simpler, but they are old, and maybe they will be discontinued.
TDA5051 is used in different X10 application (this one is very simple http://www.fuso-luci.it/hobby/domotica_x10cent.html), but we know the limits of X10 ...