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Previously Apple's iPhone was the only phone with a Google Earth app. The Google LatLong blog post announcing this new version argues that the Nexus One is a better fit for Google Earth's 3D interface thanks to its 1 GHz processor and 800 x 480-pixel display. Google says this is the "fastest mobile version of Google Earth yet."
The Nexus One app has some features the iPhone version doesn't, notably the road overlay. Google Earth is a lot more fun to use when you can actually use clearly defined roads to navigate. One of the most lauded general features of the Nexus One is voice recognition; the Google Earth app takes advantage of that. For example, you can say "Eiffel Tower" and it will take you there.