Google updated Google Earth for iOS this week and included the ability to view custom maps. The new feature lets you click on a KML file you find on the web and open it automatically in Google Earth. KML is short for "Keyhole Markup Language" and these files contain geographic information that you can view in software like Google Earth and Google Maps. To see how it works, you can check out New York Public Library's collection of geo-located maps of New York City.
Besides KML support, the latest Google Earth version also includes Earth Gallery, a new feature which lets you explore a list of curated maps that show as real-time earthquakes, hiking trails, and city tours. It also adds a few stability and performance improvements.
Google Earth is available for free from the iOS App Store. It's a universal app so it'll work natively on both the iPad and the iPhone.