I have an iPhone 4, but really don't use anything beyond the regular Google Maps on it anymore. I had a few apps on my old iPhone 3g. I think one was called Topo Map, and it let you download USGS 24k topo's and store them on the phone. The other is earthscape which I haven't looked at in a long time but the nice thing was that it used the NJ 2007 orthoimagery instead of Google's lousy aerials where it looks like half of the pines were photographed at night. :roll:
Go to the app store and you will find endless gps-related stuff. I prefer to use dedicated GPS'es for my own travels, but the iPhone 4 has a much better GPS chip than the older models and the screen is fantastic, so I'm sure you will enjoy it.
Are you getting the Verizon phone? The early reviews I read said that Verizons coverage and call quality was better, but their data rate was significantly slower than AT&T.