The French IGN mapping site, Geoportail, can be accessed via your iPhone, if you can get a signal.
The devl. seems to be a private initiative that is a portail to the Geoportail site, the DNS info says that the developer is Franck Denis. It uses the Geoportail APIs [confirmed by the link below, if my French is good enough]. If you are in France try the above URL in your iPhone. It will open up a 1:25.000 series IGN map and position you on it using the GPS (if you have one). You have the usual Geoportail functionality so you can also access aerial photos which may be useful given the IGN maps are limited on detail like small cliffs.
Other maps such as administrative plans are also available, this could be very useful for architects, civil engineers and expert-geometre etc as well as walkers, skiers, mountainbikers.
More info in French.