As it was explained to me about a year ago from a buddy who works for at&t Mobility as an Engineer, domestic roaming is basically a thing of the past.
Almost anywhere an at&t customer (prepaid or postpaid) goes he or she is registered on a native at&t Mobility cell site.
In fact in those places where an at&t customer might need to roam onto the local provider due to there being no available native at&t coverage at&t does not even sell its products thus, most prepaid users would never be able to obtain their services and thus would never have the need to roam onto that local provider.
Again, for the most part, the average user will never have a need to roam onto that local carrier as there is some (being the operative word) native at&t coverage available.
Dan