I think it also depends on whats in the air tank.
if it's just regular air, it's probibly no big deal. if it's pure O2, or some other highly flamable concoction, then there could be an explosion.
and i think that letting almost any called shot hit the air-tank is a bit high in the probibility range.
the remaining lung is quite a bit larger... the air tank looks to be about the size of a heart. and remember, any bullet has got to go through the armor that the target is wearing (what runner goes bare chested into battle thease days?) the skin, the ribcage, and then, it's got to hit the tank at a perfect angle. if it's a bit off, it'll just richoshea arround some more.
for color, you could probibly have it 'cook off' like explosive rounds if they ran through a burning building, and took a bunch of dammage from heat or something, but inorder for the cyberwear to fail, it's got to take almost lethal dammage in and of it's self. (a single wound inflicting S+ dammage in a single shot is what normally qualifies for a body resistance test to see if anything important was broken right?)
so, they'd have to take S dammage from fire / gun / whatever before it'd probibly exmplode. (at that point, an additional S in dammage from an exploding airtank is just mean)
I'd probibly leave this more as flavor text on recently killed PC's / NPC's. You shoot them, and they die horribly in that tragic car wreck... and then... explode! oops... must have cooked off the air tank.
or let the PC's use it as a goodbye 'frag you too' when they buy the farm, and want to go out with some style.
most cyberwear has it's own monitoring devices, if it deteced a rapid pressure loss, it could probibly open up the dump/refill vent and push out whatever was inside in a less destructive manner than exploding. (again, that's mostly dependant upon whats in the tank... if you take the S dammage from fire, and are in a burning building, having your Hydrogen tank dump it's contents out the drop valve isn't going to help much. you'd have just installed a one shot flame thrower into your neck.)
*shrug* that's my thoughts on the subject.
-RuskiFace the Pirate