Coverage? (US$0.99) is a new iPhone/iPad app that will let you see detailed cell phone coverage maps for all four of the major US providers. The app can find your location, then show you AT&T coverage. Even better, it will allow you to also see Sprint, Verizon and T-Mobile coverage, and you can overlay any combination of coverage maps over your position.
If you're traveling, you can show any location in the US, and you can also see what the good or bad news is for other destinations. Of course, that is the depressing part. Seeing the Verizon and AT&T maps together for where I live in Arizona isn't pretty. You can compare the 2G maps, 3G coverage, or even where there are roaming agreements. In almost every comparison, Verizon simply looks better. In southern Utah, where I spend time doing photography, the comparisons are even uglier.