It seems like every advance in technology brings its own set of health concerns, and the recent explosion of smartphones and touchscreens is no different. An article from InfoWorld details the various ergonomic risks of using the latest mobile devices, but they're mostly the same Repeated Stress Injuries (RSIs) we've been dealing with for decades now.
With computers, good ergonomics come from positioning the screen and keyboard to minimize eye-fatigue, neck strain, and stress on the fingers and wrists. However, touch screens make such placement difficult because the keyboard is on the screen — you can't place it both vertically for easy viewing and horizontally for comfortable tapping. In addition, the lack of tactile feedback on a touchscreen can supposedly cause us to exert more pressure on our fingertips, and hovering for an anticipated tap creates isometric tension in the tendons and muscles holding your finger ready to strike — both of these can be stressful. Data is still inconclusive on how much trouble touchscreens can really cause, but it doesn't hurt to maintain good posture and avoid any straining or awkward positions with your wrists, hands, and neck. Also, taking a break is never a bad idea; we'll probably survive without poking a screen for half an hour.