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.
Since the maps are built into the app, you don't have to download them, which is a good thing, since in many places you wouldn't have good enough coverage to see them. The app is 14.9 MB, by the way. If you don't want to pony up the $0.99, take a look at the free Cell Phone Coverage Map app. It's only good in big cities though, where you may have better reception anyway. Of course, AT&T has some issues in metro areas too.
Coverage? is a really useful app. It will be used on every trip I take, and even around town it shows why some calls may drop. Ultimately, and sadly, this app is a great marketing tool for Verizon if, as rumored, they do get the iPhone next year.
Coverage? is a universal app and requires iOS 4.2 or later.