Hantto: Not hard at all using some off the shelf components. Have a look at the HT12E and HT12D. The 'E' is an encoder for RF transmitters, encodes binary and an address. 'D' is a decoder and takes a signal from a Receiver and decodes that if the address matches what is set on the pins. The output is binary and can easily interfaced into a circuit.
The RF transmitter/receiver could be the following units:
**broken link removed**
I built a circuit using these and very few other components, the range using a 1/4 wave aerial is up to 50M indoors and 250M outdoors LOS.
So the answer is, it's as difficult as you can make it.
Enjoy.