All network providers use roaming partners in certain areas. This allows all the providers to allow customers to have national coverage, without having to pay for licenses and the building of a network that spans the entire country. For instance, on the Verizon website, there is an option with their coverage map to show only the Verizon network, and it has many more holes than their 'America's Choice' map where practically the entire nation is covered in red. For Alltel, this is their Native Coverage Map. This shows areas where Alltel sells service. But, they will also provide service through other providers in areas they don't cover. That is why, under their 'Smart Choice Packs' map, the map covers practically the entire country in Blue.
This website might help a little bit if you want to understand the details of it: PRL Interpretations
But basically, for Alltel data, your phone would scan for signals in this order: 1. Alltel EV-DO, 2. Sprint EV-DO, 3. Alltel 1x, 4. Sprint 1x, 5. Verizon 1x, 6. Other Smaller Providers 1x.
For voice, it becomes simplified, 1. Alltel, 2. Sprint, 3. Verizon, 4. US Cellular, 5. Other Smaller Providers
The roaming list is different for different providers, but there are also many places where Sprint and Verizon would fall back onto Alltel, as Alltel is the main roaming partner for both of the other providers.
I looked up the cities you mentioned and in Tyler you would be on Alltel EV-DO. In Charleston-Alltel 1x (EV-DO later this summer). In Chicago, Des Moines, and Dallas-Sprint EV-DO. And in Galena-Verizon 1x. There is no roaming charges for data or voice anywhere around any of these areas. Once you leave your home area, your phone may blink roaming, but on a Smart Choice Pack, you will have access to all of these networks without any roaming charges. And it is a little confusing, because you may be in Charleston, and your phone may say roaming even though you're still on Alltel, because you're not in your home area, if that makes sense. But Alltel designes their roaming indicator like that for people on regional plans so they'll know when they are out of their area.
Sorry to write a book, but I hope that makes sense and explains the process well enough and I didn't get too confusing on it.