Woo, multi topic thread! Awsome.
Well, might as well toss in my 2 cents.
First off, i think ventalation, while it would be cool, should be a verry low prority. Perhaps even a DF 1.01 development thing. It is horribly complex to track, and brings verry little to gamplay. In my oppinion the lighting arc is a way higher priority, and even that is a fair way down on my personal list of what DF needs.
If others dissagree then i think we need to keep it simple, so we do not have to wind up simulating a gas exchane system. A simple vent structure, a fan by open air, and a pipeing system that airiates a large set volume, that flows out from the origen point. So a fan can ariate 1000 tiles, or whatever, and it is a simple volume fill calculation. Fans can be placed on any fresh air tile, and propigate that fresh air, like a repeater. When calculating the fill pattern, tiles that are already fresh will not be counted, so 3 fans placed right next to each other will fill a (contiguios) volume of 3000 tiles. Pipes will deliver air to where you want the fill to start from. Stail air would not be deadly, but would slow down work and cause significant unhappy thoughts. Only special "deadly air pockets" would be lethal. These would only form, with a very low spawn rate, when there is not a fresh air tile within, say, 200 tiles. They would spread and dissapate like misma. Open air tiles would also count as a fresh air source, but would only fill 500 tiles or so.
We would also need a view mode to show fresh, stale, and potentialy dangerously stale air.