Nick, apps like "ITopoMaps" let you download the map segments for where you're doing the hike ahead of time, then when you're out beyond cell reception, if your GPS is working (and yes, darn it, it will also be looking for Edge Cell Receiver but won't find it--if you have disabled 3g, disabled wi-fi (but you can't disable Edge without disabling GPS)), you'll see where you are at with just GPS reception and the prior-ahead of time downloaded map segments. Downloading the map segments is best done when you have 3G or wi-fi reception.
Tom Harrison has many of his maps, including the JMT Map Pack, available as Iphone Apps which are downloaded ahead of time, just like ITopoMaps.
I recommend everybody who wants a GPS-only mode, to bug apple. I believe email@example.com is a valid email address for Steve Jobs. I believe emails there are probably randomly read or pre-viewed by his staff.