I know what you concern about the macbook air. I also have the exact concern on choosing my new laptop. I can clearly tell you the headphone jack of the new macbook air is dead silent all the time. I had a macbook pro 13 mid 2010 base model just before I bought the air. I used it for 3 months and the headphone jack was silent when no sound output from the laptop. It had the very very low level of white noise in the background once I played any song or sound output from the laptop. And I could even hear the white noise suddenly stop seconds after the sound output was stopped. Which means the noise didn't start and stop at the moment the sound output came out. The noise actually start before the sound output come out from the headphone and stop some seconds after the sound output stop. This is disappointing when happen on macbook pro.
To conclude what I just said: the macbook air's headphone jack is like those in iphones or ipod(dead silent at the time).
In the other hand, the macbook pro 13 still have the flaw on the headphone jack noise control.
Awesome! Thanks for the info!