hi elementalist,
I've got X10 lights in my living room (2 sets of ceiling downlighters, 1 set of wall lights, and a mains powered uplighter. I control all of these via my pronto and have macros setup to go to specific dim levels for different things.
X10 is a mains based signalling technology, the way it works is that you plug a small infrared receiver into a normal mains socket, this reads the IR signal sent by your remote and sends a signal over your mains wiring in your house, this signal contains a few things: a device code to tell the switches which ones its talking to and then a command (such as dim, off, on, all on, all off, etc).
IIRC the IR transceiver costs about £30 as does each light switch, the plug in module cost about £20. have a look on
www.letsautomate.co.uk for more accurate pricing.
The system works very well and is quite reasonably priced but there are a few things to bear in mind: certain appliances will interfere with the X10 signal if they are near to the light switch you are trying to control (tumble driers, some fridges and certain low voltage halogen transformers). From my experience of this there isn't that much of a problem, the only thing that seems to bother my setup is that sometimes if the halogen lights are on in the kitchen one set of downlighters won't respond, all the others work whatever. Our fridge, washer dryer etc are all fine it seems. With the X10 stuff you buy in the UK you can't have the lights come on from dim, they come full on and then go down, no the end of the world but not quite as professional as some of the more expensive solutions.
As far as fitting goes I found no problem, the light switches just replace your existing single gang switch, no extra wiring is needed, they just screw into place, the transceiver and appliance module (the thing that controls the mains uplighter) just plug into a normal mains socket.
HTH, any more questions just ask,
owain