I agree with everyone on the map thing -- though for the JMT it can be a large scale one for the whole trail or the PCT one.

A GPS is useful on snow when it's not clear where the trail/route is. Also they're kinda fun. The Garmin 60CSX is a pretty good and inexpensive GPS with a long battery life but, like everything else, you've definitely got to learn to use it. The Garmin maps are expensive. As a test, I've used the free maps from www.gpsfiledepot.com/, then search for California. The set I used is California Topo 2011. They're not quite as good as the Garmin ones, but definitely useful as well as free. So whatever GPS you get, make sure it has an SD card slot.

Also, I'm not a phone geek. Does the iPhone GPS work without being connected to either WiFi or a phone network? I was told it doesn't, yet I've been sent very accurate position fixes via google mobile maps from a wifi-only iPhone 4. They are not the location of the wifi, but almost certainly of the phone.

g.

Edit: This link is a little more direct to the California Topo maps:
www.gpsfiledepot.com/maps/view/28


None of the views expressed here in any way represent those of the unidentified agency that I work for or, often, reality. It's just me, fired up by coffee and powerful prose.