Okay, so JMA's newest pre-quake warning system doesn't cover as much ground as SMS alerts, but with only 10 to 20 seconds of available panic time before an earthquake hits, we'd probably be toppling over one another before we could even find our cellphone anyway.
Some of the reasons cited are shortages of key components in part due to the Japanese quake and unstable performance of Android Honeycomb operating system, which Google specifically developed for tablets.
Since the primary waves travel more quickly than the comparatively sluggish secondary waves, the system picks up on the magnitude and speed at which the quake is approaching, and beams the data over fiber optic networks to emergency relay stations setup in corporate towers, governmental buildings, hospitals, and transportation venues such as railways.