I have to admit that I've somehow missed iNavX until now, because it is available trough the US App Store at this time.
Anyway, iNavX is a chart plotter for marine charts. Not only plots it your position on the chart in real time, it also provides navigation info like heading, speed and so on. Waypoints are also supported, of course.
The maps used are being downloaded from the NOAA servers but then stored locally on the iphone. There is no way to download your own maps for usage on the phone.
It's nice to see that, maybe in the future, there might be navigation apps for outdoor activists, that use topographical maps. Perhaps something like the Ozi Explorer CE, who knows?
At this time, it seems likely that you will have a "pair" of applications, one to prepare the maps and data on your computer and the other to display them on your iPhone.