Google has a released a custom, multitouchy version of Google Earth for the iPhone, and it rocks, turning you into a master of universe that can zoom, twirl and pan all around the world with just a few flicks of your fingers. It also uses the accelerometer, so you can pan and tilt, too. It might just be better than desktop version just because multitouch makes it so damn fun.
Overall, the whole thing works really well. Performance is good, at least over Wi-Fi, though it can get occasionally janky. Google has overlaid areas of interest with Wikipedia and Panoramio links that pop up with info or sweet photos in an in-app browser, in addition to giving it the iPhone-standard "locate me" GPS powers to find stuff around you. It's free, and zipping around mountain ranges with a fingers like a demigod just 'cause you feel like it is awesome-o, so definitely grab it now. [iTunes]