My first question would be how do you terminate a DC system. In other words, do you just pigtail the wires from a multitude of J-boxes or is there some sort of receptacle you could plug into. Might look funny with 12V car cigar lighter style jacks all over the place.
I suppose I would run one branch to ceiling recessed j boxes and wire nut them until needed. I would run another branch to anyplace you might want to have a PC or networking gear. Separate branches so you can use different voltage if need be. If I had to pick a voltage, I would choose 12V. However, 5V from USB style jacks seems to be an up and coming standard that might be worth looking at.
Sure most things ultimately run off of DC but what DC do you want 3.3V, 5V, 12V, 24V or even 48V (new power over ethernet). Notebook PCs run off all manner of oddball Voltages and external HDD, Routers and modems run off 12V, 5V or even 3.3V. Cell phones charge from a variety of voltages too.
DC-DC converters are complex and somewhat expensive when compared to tried and true AC-DC linear and switching supplies made in vast quantities with reasonable prices (not to mention they usually come with the applicance)
So if you stick with just lighting applications, you can't really count on the voltage being standard there either. All sorts of new driver chips and switchers designed for AC will come with those lamps. You would need to survey the market and get the lamps when you choose the supply voltage. The high brightness LEDs usually are driven from special driver chips (presumably PWM) and not steady state DC supplies.
Its a great idea but the infrastructure and technical standards don't really exist yet. As such you would be swimming upstream and destroying your cost savings by buying things that may not be applicable in the future. Without code standards, you could end up with a White Elephant, it wouldn't necessarily improve the value of the house since the next owner will be so confused as to not use it.
If you want to run extra wires, stick to A/V. Whole house audio and room to room video is useful but even there, what wire to use Composite or HDMI and when wireless video comes of age, what then. Come to think of it, 12V would sure be handy for Car Audio equipment, you could save a lot using a car amp but #14 wire won't do it there.
Before you jump, try attending some of the many webinars on the LED lighting topic. Design news T&MW, EDN, ECN, PD&D (and others) magazines all have specialty e newsletters for LED lighting or ones covering the topic with others. Most of the semiconductor companies also have multiple chips for driving and LEDs for lighting complete with white papers, app notes and datasheets galore. Its very early in that industry now, a wild west scenario for those trying to sort it out now. Good luck.
In the end, you might be better off running empty conduits and pulling the wire later when the choices are more universal. Grey PVC is reasonably priced and easy to work with as a DIY project. Just be sure not to let it become a mouse or bug highway.
One final Note. Title 24 is a must read for anyone remodeling or building a new house, even if you don't live in CA
http://www.energy.ca.gov/title24/
http://www.energycodes.gov/
RGBW LED to experiment with.