Since the actual process of the transaction is instantaneous, and its takes the money directly out of your account, the account they're dealing with is most likely Revenue.
Accounts Receivable is also another option that may come to mind, but remember that in this account, the seller is waiting for payment. Once the responsible party pays the seller, A/R is credited (decreased) and Revenue is debited (increased).
With iTunes (as stated previously), the transaction happens right then and there. We pay cash and iTunes gives us the song/movie/album/etc. Therefore, the only logical answer would be Revenue. In this case, Sales Revenue since we're dealing with a type of retailer and not a service.
