Or you could get the 3G Verizon iPad 2. You have an iPhone 4. Pay for the hotspot on the iPhone 4. When you need data on the iPad in a good AT&T area you can use that.
The 3G on the iPad 2 can be turned on and off month by month right on the iPad. It does not require a contract. So when you get to a place that AT&T isn't working well, just buy a 3G plan on the iPad. When you are done you can turn it off with no penalty, right from the iPad.
For quite a while I was doing it the other way around, with AT&T on the iPad and Verizon on my Phone. It works well, except I never got around to turning off the iPad's plan. It was just to convenient to have the data whenever I wanted without fumbling around with the phone too.
I only used the phone when I wasn't near a WiFi spot and 3G AT&T was not available.