XRoad's G-Map is a 933MB app for your iPhone that gives you offline GPS for anything West of the Mississippi. Too bad its average review is two stars.
The app itself is $20, and can give you directions even without Wi-Fi. It's got standard GPS features like point of interest, location searching and 3D view, but according to the reviews, it sucks and the developers should be ashamed of themselves.
Some of the more major complaints are the lack of voice navigation, no instructions, limited POI, constant crashes, and no 3D. Weird, since they actually show a 3D screenshot as one of the sample shots in the App page. This may be the first GPS app for the iPhone, but you should stay away. [App Store]