Be careful how you define "efficiency". Yes, not inverting DC is more efficient electrically, but that does not always translate into you getting more of what you want for how much DC you put into it. For example, I challenge you to find a DC blender that will be as efficient as a cheap AC one driven by an inverter, and DC CFLs are quite expensive and there is very little selection available.
I want my refrigerator and some lights to be DC so a failed inverter doesn't put me out of commission, but using AC for many other things will save you money unless you are very frugal and forego most of the stuff most USians take for granted. If this is where you are on your personal path, I commend you; however, be honest with yourself, get second opinions, and do much research before you decide to forego AC entirely.
My recommendation is to have both AC and DC runs in your house and use the most efficient one for each appliance you have. This will also make it easier to pass code; in fact, why don't you make them all AC, pass inspections, and then change the ones you want to DC ? No questions asked is a good way to go.
For a DC HVAC system that will pass code (i.e., is UL or CSA listed), good luck. I have heard of people putting in one or two electric baseboard heaters because this is the cheapest thing to do. The electrical inspectors don't even care that your RE system could never in a million years power it. As long as it's stamped (UL, or whatever) and they can check it off on their list, they are happy.
Don't get me wrong, I don't think all electrical inspectors are no good. However, even the good ones do not have the time to research every non-standard thing you present to them, and once they see some things that are questionable in their experience, they will look all the harder at everything else, which you do not want. If you do decide to go all DC, have a long talk with the actual inspector who will do the work before you even get started. If you have to change things mid-stream, it will not be an enjoyable experience.
Best of luck,
Dave