Bigger than a 7" screen and the device becomes "transportable," in my view. An iPad or laptop might work fine in the vehicle, but you're not going to take it on a hike to the top of the mountain.
On the other hand, the small pocket GPS receivers from Garmin et al are not that expensive. Would it make more sense to have two devices, one with big screen for use in the car and in camp, and one that fits in a pants pocket for hiking/biking?
I currently use a few different setups, depending on what I happen to remember to bring me...
10" laptop in the car (Lenovo S10-3t)...that's a netvertible/tablet/whatever, way to big for backpacking...however it has all my 'master' information on it...
For true backpacking, I bring along my Garmin 60csx, I can d/l maps from my S10 either ahead of time, or in the field if I change my plans....it's small, has a decent screen, and work as a real gps...
For minor backpacking, I use either a smaller smartphone (fuze, tilt2, bb8900) or my ipaq 4700 (old school 4.5" screen), and load in my maps (actually have just about everything stored on a micro-SD card).
Honestly, I've been messing with this forever, and I haven't been able to find a good compromise between portability (iphone, fuze, tilt, ipaq, borrowed EVO) and legibility and real-life functionality in the truck (notebook, netbook, tablet)...I used the ipaq for years as my mapping GPS (I originally loaded pocket S&T 2003 on it), and IMO there's no easy medium...get a screen that
is small enough to cart around and it will be a sacrifice in the car, get a screen that's truly functional in the car and it's too big to go in the pocket
I think, no matter what the technology, neither apple, nor samsung, nor dell, nor anyone else will solve this problem. They may drive some short-term demand because it's 'cool', but in the long run, it's just not practical (until they come up with rollable screens, then I'm all in!)