Most cell phone GPS units don't require cell service, however they will (sometimes) take longer to locate you since they are designed to use cell towers to triangulate your position to locate the satellites faster. They will lose signal faster than a standalone GPS (I can verify this through my own experience). The new Samsung Galaxy S 2 (I think that's the name) is supposed to come with a SirfStarIV GPS unit, which would give it similar capabilities to standalone GPS units.
That said, I've used my Motorola Atrix as a standalone GPS hiking outside cell service and it works surprisingly well.
One thing to consider is that a lot of mapping software (google, bing, etc) require a data connection to download maps real-time. Some software allows you to download ahead of time (preload) your maps, essential if you are outside cell range and want to use your phone.
I have an iTouch, a Blackberry, and an Atrix. I've been trying out lots and lots of 'backpacking' software on the three, and have finally settled on using the Atrix (Android) and Backcountry Navigator. I can download maps on my desktop, then load them onto the phone. I can also download the maps on my phone, but the desktop allows me to download more at a time faster (and a much bigger monitor).
My wife has an iPhone (plus my iTouch) and I have yet to find software that is as easy to use as backcountry navigator for android...mostly around the 'download on computer and load onto phone' but also, it seems that the iOS apps aren't as refined. Just my perspective...and Blackberry is pretty much useless (it's my work phone, so not a big deal for me anyway).
One huge caveat....I went on a short hike outside cell service last weekend...had my Atrix (with built-in GPS) and the iTouch (using my SPOT as GPS), and just on a 2-3 hr hike with the GPS running on the Atrix and Bluetooth running on the iTouch, I was around 50% battery at the end...even with turning the screens off. I would use my phone for short hikes, but for full-time backpacking, I'd almost rather use my Garmin 60csx (soon to be Montana
). I do have a backup battery for my Atrix, but honestly I can't see it working for more than a 2-3 day backpack trip before the extra batteries required become an issue. I'm guessing it would be the same for an iPhone, or any other phone out there.
If I had a budget and wasn't an ultralite hiker, I'd go with a good, but basic smartphone (on contract you can get them for like $100)...and then get a used Garmin eTrex or something like that on Craigslist for around $50-75...I'd use the smartphone for day hikes, and the Garmin unit for longer trips.
(as a note, I've had success using an Energizer Energi to Go pack as backup for my Atrix...if I didn't want or have a standalone GPS, this would work well to keep my phone going and charged for a longer backpacking trip...at the expense of battery weight, of course, but a solid choice and mine work great)
