Its difficult. For email and web its fine. I don't think its up to the task for HD streaming or a large library of losless recorded CD.
I have a Imac in the back, and my Itunes is on a NAS. I did not when I bought it that I wouldnt be able to do Itunes but can live with that as I only need it to rip new CDs and transfer to Ipod.
I would take a very carefull look at the 13" Mac pro at this point ... I havent seen the latest but believe the display is ages better then the blackbook I compared the air against back then.
The added lbs weight you will most likely OK with when you travel.
The Air is in my opinion too much of a compromise, I bought mine simply because there wasn't a 13" Pro back then.
BS9000, BS2300, BC2, BL2500, BL3, Bl2, BS1, BV8, BC4, A8