Notify arduino over powerlines
Sending signals reliably over the mains wiring can be quite hard, especially if you are trying to impress the signal on the active wire. You can look up and read lots of information on the architecture of X10 and get lots of good ideas for coupling signals onto the mains. You could even use most of the X10 protocol to implement your schema. I have used the Firecracker (RS232 --> RF) and TM751 (RF --> AC mains) in the past successfully, though not as a transceiver.  

Coupling to an Active wire can be problematic if you have multiple phases in your house (perhaps not likely for you adjacent room scenario). You have to then provide coupling between phases (even the US bi-phase is a problem here) to ensure the signal emanates to all points in your wiring. 

You can avoid all the hassle by using just the Neutral and Earth wire. These wires are not phase sensitive and are both bonded to local ground near the junction box. I have used many powerline Ethernet devices (most couple to Neutral/Earth to avoid phase problems) to achieve this, and you could simply use an Ethernet Shield on your Arduino either end (you still need to provide the interface to the Earth wire of course) and you'd be good to go.  

If you want to build something much cheaper, then I have used the 2.4 GHz NRF24L01 with a PA to couple signals directly onto the Earth wire. You may need to put a couple of Ferrite noise suppressors on your local ground connection wire at the earth point. (Like these). I just wrap a few turn of wire from the NRF24L01 PA antenna socket onto the earth wire at each tranciever. 

You could also consider current transformer coupling onto Neutral/Earth, and that is nicely explained in a Google patent.