Me too - standard 12V and 5V rails run throughout the house would be great. I even thought about a wallpaper with conductive strips so the power could be invisibly delivered to any part of any room and "tapped" with a push-into-the-wall socket.
The usual counterargument is: voltage drop can become a problem. Trying to use one big power supply and use DC as your only distribution mechanism probably isn't a good idea.
But choosing a DC system for part of the house can make a lot of sense.
For one residential new construction room, it can be practical to have one shared power supply rather than one per LED. Say you have a 12 V, 5 A DC power supply. Using a star wiring topology, this can serve 10 lights (at 500 mA) fine with 16 AWG.
But how far can the power go before wire resistance causes too much Vdrop? Maybe one good transformer+rectifier per room? AC to the room and DC in the room. Those DC runs would be <5m each.
