RF network using Arduino
The NRF24s are great, but limited to communication only.

Digi's XBee 2mW module has got great indoor range, but more to the point it's got 8 I/O lines and 6 10-bit ADCs on board. It's also built for mesh networks (or you can get star-network equivalents) and has security encryption to boot. This means that, for some of your nodes, you don't need any other hardware to have a fully functioning control point (not including relays and sensors of course), so you can ditch having arduinos at every node.

They're so easy to use, very low power, and have plenty of support and documentation available online.

Do it!