Ok... not smoking per-se, but the 12V plug they built into every car to run the cigarette lighter.
Many of the electronic items in the home do not require 110V AC. If it has a big plug adapter or a power brick attached, the item is converting the 110V AC down to some smaller (something around 5 - 12V) DC current. Those conversions are not perfect and energy is lost.
What if you recovered those losses by having a single efficient conversion instead of many less efficient ones (or required that all converters be more efficient)?
Assuming that computer power supplies are representative of overall power supply conversion efficiencies, the average AC/DC converter is about 75% efficient. On the upper end, you can get ~90% efficient converters. So say a 15% difference in conversion efficiency between several average converters and one efficient one.
According to the DOE End Use Consumption of Electricity Survey (2001 - latest data), about 15% of a house hold's electricity usage goes to things that could, plausibly, run on 12V DC power. That was about 173.5 Million MWh of electricity. Improve that by 15% and you save 26 Million MWh of power.
The average output of a coal fired power plant is about 233 MW. So, for simplicity, if you assume 100% duty cycle, that 15% efficiency boost becomes the equivalent of 12.7 coal fired power plants.
While that is maybe not world saving (<1% of the 1445 coal fired plants in operation in 2008), that is ~5.5x the total Solar (thermal + PV) generating capacity in 2008... a bigger impact than solar has gotten us so far.
Where DC wiring, rather than just more efficient converters, might be more important is in Solar PV on homes. In that case, the DC output of the panel is converted to AC before being used in the home. That step has about a 90% efficiency.
However, many appliances then convert the AC right back to DC with all the losses noted above. Even with 90% efficient converters, that's a 19% efficiency loss over using the electricity directly from the panels.
Viewed another way, the AC wiring in your house is raising the price of Solar PV by almost 20%!
photo credit: David Baker (Flickr)