Since NOAA vector charts can be used, it seems it should be possible to make an application that reads "garmin" format maps also. Maybe a solution will come from the russian creators of alternative apps for windows mobile (I.e the author of GPSmapedit & Co.)
So far, there isn't much hope yet for a road navigation app with lock on road, but already an application which allows one to see one's position on marine charts and topo maps would be great.
It's OT but cool, go to their OnLine maps and download US chart overlays to Google Earth. As you zoom in along the coasts, the charts appear (on desktop). Don't think the iPhone 3G Google earth app handles that yet.
There is no reason to believe that Garmin will provide a reader for its format on the iPhone, since that would decrease interest in their own Nuviphone.
So far, I haven't seen anything published on apps for reading img maps. It is possible that img format maps could be converted into formats that can be read by eg the Navimatics app but that seems to be a clumsy solution.
Another company, Navionics, is maker of the well known navionics vector charts use in many marine plotters. They announce iphone apps as coming soon:
navionics won an award at Mets for their smartphone chart application
and, as mentioned, they've posted a free viewer and map on Itunes under navionics.
These two examples show that it's feasible to view and use vector maps on the iPhone. Actually, the free navionics map of Italy looks pretty much like a garmin map. Mapping data for land is from Tele Atlas.
Then of course, Tom Tom had a road navigation application ready in July but was rejected by Apple.
iPhone 3G, iQue 3600, Loox 560, Garmin 60 CSX etc.