Say you have an iTunes account setup on your computer. If your iPhone uses the same Wi-Fi as that computer, then the purchase will be charged to your iTunes account.
However, if you are using Wi-Fi that does
not have an iTunes account set up on it, such as Starbucks, then the purchase will be charged to your bill.
BTW: I was at friends house the other night. He has Wi-Fi with his iTunes account on it. I downloaded a couple of songs courtesy of his account...
