If the car goes into the garage, you are pretty much at their mercy whether they find anything or not. You might want to try getting a generic scan on the car before taking it into the shop. You may be able to do that at a parts store for free. Ask for an OBD scan. Generic OBD canners are cheap and a great way to help troubleshoot.
Lastly, if the dealer tells you your car needs a new brain (ECM) or something, go do some homework.
The obvious "is it plugged in" question is about the wheels that open and close the vents. 1 is open and 0 is closed. When the wheel is in the 1 position, can you see into the vent?
A couple of other things to try before you take it in to the dealer. First, do you get air from the vents in any conditions? Is there heat but no A/C? A completely whacked A/C system in a BMW will blow warm air all the time unless you physically close the vents. This is a "safety feature" so that if the system fails and you happen to be in antarctica, you won't freeze. Do you get cold air at the windshield or at your feet but just not from the main front vents? Does the"Auto" button on the AC get you something different than the individual manual settings? (I know you've got I-drive but you should still be able to set airflow manually.) OK, next thing. When you change the airflow around in the car (feet, windshield, front vents), do you hear dampers open or close on both sides of the car? Your car has two-zone (driver and passenger) A/C. If you hear dampers on one side and not the other, then it is likely that one or more damper motors are not working. Those things are a pain to get to and best left to a dealer, but if you can hear them working on one side and not the other, then you have a really good starting place. That's about all I can give you to try at the moment. Please report back on what you find out.
I hope this helps.
Current cars: '99 E39 wagon (200k+), '99 E36 Cabriolet (220k+), GMC Yukon Denali (186k+, but who cares?)
Prior BMWs: '99 E39 (120k), '94 E34 (220k), '93 E34 Touring (320k), '86 E30 (275k), 76 E12 (175k), '74 E10/model 115 (2002 tii) (110k)
BMW's run forever and the Internet is a nearly infinite resource. Use them both wisely for maximum enjoyment and return on your hard-earned cash.