I'm posting here since I was unable to get a satisfactory answer when I called customer service...
After 5+ years as an AT&T wireless customer, I made the decision to switch to a different provider. I arranged to have my service terminated and my number transfered to the new provider on the last day of my AT&T billing cycle. The process was complete by the first day of the new billing cycle, and AT&T stopped providing service on that day.
I was subsequently billed by AT&T for a full month of service, even though (as previously noted) my AT&T wireless service was terminated by the first day of my billing cycle.
I was aware that AT&T does not prorate- but I find it extremely frustrating to be billed for a month during which I received no service. The customer service representative claimed that the reason for this was that "the final port was not complete prior to the beginning of the billing cycle," but to me this is an unsatisfactory explanation for why I was billed after my AT&T cell phone had ceased receiving service.
Definitely something that I will remember to tell anyone I know who is considering signing up with AT&T.