First of all, thanks for the post Jesse/tacedeous!
Second, I'd never heard of this Garmin XT before. Sounds like it uses the MiniSD slot. I wonder how it compares to some of the excellent GPS apps out there in the last eight months since you posted. Would love to here more!
Third, My primary reason for contemplating carrying a smart phone:
== CONVERGING DEVICES INTO ONE ==
I've been contemplating getting an Android phone for a couple months now. I've skipped the iPhone because it's a locked down POS... primarily this is because ATT coverage is abhorent in my kneck of the woods, but also Apple's nanny-like approval process for all apps.
The reasons I'm planning on doing this are simple.
Mostly it turns four separate devices into one.
1) replaces my camera for video and photo (I carry this everywhere)
2) obviously it's a phone (I carry it everywhere purely for safety's sake, but it remains off)
3) no need to carry a GPS (up till now I've simply refused to carry one)
4) iPod (I've stopped carrying one in the last year because I felt it to high maintenance, though I do miss it on occasion. It's good for raising spirits on those rare occasions when you get in a funk)
So, at it's most basic a smart phone replaces two key devices with one. If you count those I have neglected to carry because I despise the clutter then four.
== HOW I PLAN TO USE IT ON THE TRAIL ==
First of all, I should explain. I don't just hike. Mostly I bike tour. I'm planning on combining the two this summer to section hike some trails. Maybe / hopefully even some parts of the AT or possibly a larger section of the North County Trail.
Basically that works like this: I have the base weight for ultralight road bike touring setup below 30lbs. This includes gear AND bike. (picture: http://www.flickr.com/photos/mmeiser2/4245474921/ ) With a harness this should allow me to hike with it on my back simply by removing the wheels. Thus this should give me unlimited freedom to explore hiking trails without having to drive or circle back to a parked car. (More Freedom!)
Admitedly this approach is a paradigm shifter. It's impossible to know how well it'll work and just what the advantages are until I've tried it extensively. However, if you choose to you could think of this approach as a great way to scout sections of trail for a future thru-hike. Isn't that what all section hiking is. ;)
So, without any further adieu my somewhat prioritized reasons for carrying a smart phone.
1) web, web, web... There are an infinite number of reasons why I might end up accessing the web but the best example is the simple ability to google the nearest grocery store or gas station (or bike shop) should I have an issue or a need to resupply.
This is integral to my approach.
I don't care for food drops when bike touring, don't need them, and when bike/hiking I shouldn't need to carry more then a few days food when potentially at any road crossing I have quick access to resupply. Hence I can go lighter then most backpackers to compensate for the 19lbs of bike weight. Thus the equation shifts from not how much do I have to carry but simply a matter of how long would I like to avoid civilization.
2) taking photos and video (and posting them straight to the web)
3) GPS tracking, mapping, route following. This as opposed to google maps... I distinguish between the two because I can use GPS maps when offline.
4) blogging / micro-blogging (twittering) / journals... I find it's best to say it with video or photos rather then do to much typing.
5) music... plus audiobooks and podcasts
6) phone... mostly it's simply for emergencies
So, that should be it