My son, a personal trainer, told me that one of his clients was complaining of neck pain. He asked her how often she used her new Kindle Fire. It turned out that excessive tablet use coupled with poor posture was the source of her pain.
When using a cell phone for text messaging, anchor your upper arms close to your body and bend your elbows to raise the phone closer to your face. Then lower your eyes, instead of tilting the head down, to see the screen. Avoid bending the neck forward and down; if you still must tilt your head, use the top-most “hinge” joint of the neck. But do try to have your arms and eyes do most of the work.
A handheld tablet with keyboard seems like it would be very awkward to type with (needs thumb-typing or one-handed typing). So a tablet is probably placed on a surface for that. My son says the Apple Ipad has dictation capability; maybe typing is not so necessary. Then there’s the stylus, a one handed implement for note-taking. [update: one needs to speak very clearly for Ipad dictation to be accurate (good speech therapy for those who need it!). My son bought a case that tilts the Ipad up so he doesn't need to tilt his head down as much. He also says a case is available that comes with a separate keyboard and also elevates the Ipad. Essentially the Ipad becomes a desktop.]
Typing on a laptop rather than a tablet should be easier on the neck, as long as it is on a surface like a table top, which it seems most people do. But still the monitor cannot often be raised to comfortable eye level so see people bending their necks down all the time. As with cell phones, it is better to roll the eyes down. Better still would be if the laptop screen could be separated and hung at eye level. (Do any laptops come with separatable screens?). For now I am sticking to my desktop computer. At least the monitor can be adjusted to a comfortable height, which for me is just below eye level, so I can see it in the reading part of my eyeglass lenses without tilting my head up. (First I adjust my chair height so my feet rest firmly on the ground and my lower legs are perpendicular to the ground. If the monitor needs to be higher, I use a large book or two to raise the monitor further.)
For an informative video on improving sitting posture to avoid neck and shoulder pain due to prolonged desk work, computer and mobile device use; see Sitting At Your Computer on Dr. Robert Kelty’s Chiropractic Wellness Center Website