I asked around at work a bit, and there are cases where you can roam on AT&T's network. These cases exist in areas where T-Mobile has not, cannot, or will not build out a network. In these cases, they contract with AT&T to provide their customers with network access. Some phones will show roaming, but I think the software, at this point, has made it a seamless experience, and you don't even know you are attached to AT&T's network. For billing, purposes, if you have a national plan, you are not roaming.
This doesn't help you, but I wanted to clarify my blanket statement above.