Practically every Starbucks, McDonald's and many other fast food chains have free WiFi. You don't even have to buy anything, just stand outside with your bike. With Skype on my iPod touch, I don't even need to use a cell phone when I cross the US.
The ACA Northern Tier and Transcontinental routes go through small towns. Use backroads. There are close to ZERO Starbucks, McDonalds, Burger King, fast food restaurants on the route. The ACA routes avoid all big towns. And most medium towns. The places to connect with free WiFi on the ACA routes are very few and very far between. This is what happens when you choose a very rural route across the US instead of a metropolitan route.