i dont know if you knew this part, but if you leave the vent open and just turn the dial of the AC temp all the way to hot, the air will be redirected and you will start feeling warm air through the vents even with the AC being off.
TXterra.....do all of your other vent positions work? (face, face/foot, foot, etc....)
If the selector is set to defrost (the right-most position on the dial), you shouldn't get any air coming from the face vents at all. If you do have air coming out when its in that position something might be wrong with your vent door motor. Apparently it was a common problem with the actuator motors in many cars from the Nissan lineup. I had my motor replaced under warranty for that same reason. Air was coming out of the face/foot vents regardless of what position the dial was in.
I am not an AC specialist but we do AC upgrades at work. There is a design feature that constantly passes a small bit of air over the coils to keep them from freezing up. If this didn't happen the condenser would freeze up.
If you want to details I can ask one of the AC techs at work on Monday.