i was curious about this too not too long ago. i went to a shop and asked a tuner here in houston (chris delgado from SecretServices) and he said the IACV that is equipped on the sr20's should be a step motor type and can be open to the atmosphere. i have my motor fully built also and i just have the IACV hooked up and not connected to the intake system.
note: im still in the process of finishing my build so i havent got the motor running just yet neither have i ever started it yet.
here is a link about types of IACV's
http://www.autoshop101.com/forms/h60.pdf