Interesting idea. As you know, your breath has a lot of moisture that can freeze on cold surfaces. This might tend to clog up the tubes. Maybe there would be a way to use dry air that is warmed by your body heat. Maybe a simple pump, like squeeze bulb for instance, could be used to circulate air through tubes running up your sleeves into the body of your jacket. Or there could be an air bag inside the jacket that is compressed by your body when you inhale. I admit that might be too elaborate of a system, I'm just trying to solve the condensation problem.

I don't wait so long that the inside of my gloves are at freezing temperature so there is never at time when the moisture in my breath freezes on the inside of my gloves. There is enough heat from my fingers to keep the glove interior well above freezing.

Your gloves are made of fabric that breathes. This lets moisture escape readily. Ice that forms will evaporate(sublimate) even if it does not get warmed above the melting point. I assume you were thinking of plastic tubes. Plastic tubes will not let condensation escape. Thin tubes can be blocked up by even small amounts of water.

I appreciate your considerations but my experience in removing my gloves and blowing warm air into them and replacing them has resulted in no noticeable dampness in the gloves. My concept would only remove the necessity of taking off the gloves. If the tubes of perhaps 2mm inner diameter were placed on the back of the fingers and were thin enough to flex it should not interfere with my hand manipulation and as long as they conducted the warm air up the first joint of the fingers it should be sufficient.

you do not get condensation from your breath on your hands because your hands are exposed to cold, dry air, so it does not condense on your already warm hands( though they feel freezing, they are still at least 70f) but in an enclosed space, even with some ventilation, the fibers of the gloves will be cool, and the air will be stagnant, warmer than your hands, and saturated, so you will get condensation. I can be sure of this because it happens to me constantly when i wear a scarf, or when snowmobiling and i wear a helmet, you get condensation, and end up even with a stinging sensation, because though it is not as cold as the air alone, it is a more efficient heat conductor, and the surface of the skin can actually start to freeze.... it can really be far more uncomfortable than not wearing anything

Have you considered electric gloves?

As a bike commuter in Washington DC, I find my fingers often get cold, and this same idea occurred to me this morning on my way to work, with the addition that I'd like to string tubes through my coat so I don't have to take my hands off the handlebars.

When I googled "breath tube glove warm," your Why Not entry came up first, but third on the list was US Patent 6055670, issued May 2, 2000, for a "breath-heated insulated glove and associated method"

I scanned the patent on Google patents and didn't see any reference to condensation.

I think as long as the heated air enters the glove in the tube and leaves the glove the same way, presumably after giving up its heat, the evaporation of condensation would be small.

But it leads me to the idea that maybe air isn't the best fluid for these gloves. Maybe you could design a special coat that puts a water or antifreeze bottle apparatus with two one-way heart valves in it under your armpit and runs the tubes down the sleeve of to a quick-connect that will seal when the glove is removed. You pump the bottle by pushing your arm down and it circulates water warmed by your body to your hand and brings the cold water back to your armpit.

Probably too complex. Or maybe this could be done and air maybe could be a better fluid than water or an antifreeze. At least this way, you wouldn't have to blow and it couldn't leak.