Sorry I'm late finding this thread!
I have done a ton of testing of battery drain. I have more info than is in the article Alan and I wrote nearly two years ago: http://adventurealan.com/iphone4gps.htm
To address some of the questions in the thread...
It's out there but you have to REALLY search for it - ask me how I know -- Actually, I'm pretty satisfied that if you google the words iphone backpacking the article is right up in second or third place. So if you lose the link you can easily find it again.
All that info should be in an article on BPL and show up early in a search! -- Alan and I considered putting it into a BPL article, but opted to host it ourselves so that we could update it on the fly. The data changes regularly, so ability to stay current was important to us. We've revised it 10 or 12 times since we first published it, and it's due for a major revision since iPhone 5 battery issues are significantly different than iPhone 4.
Has anyone tested this? -- Oh my, I've tested more than you can imagine. Yes, you can use GPS as long as Airplane Mode is OFF. On any iPhone with a SIM (ATT iPhone 4, or any iPhone 5) you can disable cellular by removing SIM or locking SIM (http://support.apple.com/kb/HT1316). The Airplane switch controls GPS and Cellular in one switch, so it must be left OFF. Verizon iPhone 5 is a very fine tool w.r.t battery drain. My original tests were done with ATT iPhone 4. Recently I retested with ATT iPhone 5, and my brother and DanS both tested with Verizon iPhone5. I haven't had time yet to update the main article, but you can find the bottom-line results over here:
Even if you can turn off the SIM, wouldn't you want to switch the phone to "airplane mode" to avoid running the GPS all the time? Or does the GPS not actually drain much power with the SIM card turned off? -- This is described in more detail in the article. The GPS only drains the battery if an app or a system service is asking for location. It doesn't stay active all the time. In addition to the apps in your app list, you must disable a bunch of hidden "system services" that use Location. There is a superset of battery-draining things in the SETTINGS section of this post (I'm not sure which of these matter, but certainly WIFI, Bluetooth, SIRI, and the Location->System Services, General->Cellular all matter).
With our ATT iPhone 4, SIM locked, we have hiked for 18+ weeks with Airplane mode OFF. Baseline drain with all battery conservation settings in place is 1-2% per day. Same is true for Verizon iPhone 5. But not true for ATT iPhone 5 (as described in the other thread with my test results.)
I bet you could find out from developers if their app still uses gps when the app is closed and/or phone is locked. -- you could ask, but in my experience reporting battery-drain problems to several app developers I believe that many of them are oblivious. Best to test the apps you plan to use before you go in the field. Gaia GPS and ViewRanger were the only two apps that responded to my battery drain reports, fixed their problems, and added additional don't-drain-my-battery app settings. Gaia and ViewRanger now give you an option to get a GPS fix only on demand, instead of automatically getting a GPS read when you open the app -- that means you can study the map and make route plans without having the GPS running. See more about this in the main article. http://adventurealan.com/iphone4gps.htm
And finally, the folks at my local Apple Store (Palo Alto) know almost nothing about this stuff. When I bought my first iPhone the sales agent assured me that the GPS would not work if I was not in range of cellular signal. When I spent 2 hours with the "Apple Geniuses" last week trying to figure out why the ATT iPHone 5 drains the battery when out of signal range, I was assured by the "Master Genius" that if I turned Airplane Mode ON and also turned WIFI ON that I would get gps services, which is easy to test and is simply not true. WIFI will give an approximate location if you are in range of a WIFI signal, but the gps chip is disabled when Airplane mode is ON. This is a long way of saying that we backpackers are a very small niche market and we care about things that most people don't care about ---- Give me GPS when there is no cellular signal and don't drain my battery.
Sorry I haven't had a chance to update the article, which would make it easier for folks to sort through all this info. I'll get to it in the next month.