Consider the air going around the "intake" as well. It has to have accelerate laterally to follow the narrowing cone. No acceleration without a pressure gradient so there will be a lower than ambient pressure on the outside of the cone.
I don't think it is unreasonable to assume near ambient pressure not long after the outlet from the cone. Assuming constant mass flow throughout the cone, this will mean a higher pressure in the inlet created by ram effect in the oncoming wind which falls off towards the outlet.
Low pressure on the outside, higher pressure on the inside -> cone stays inflated.
Please find any errors in thinking or assumptions. <img src="smile.gif" border="0">
Cheers,. . /ft