The patent proposes "a material attachable to skin, the material capable of detecting a magnetic field and transferring a perceivable stimulus to the skin, wherein the perceivable stimulus relates to the magnetic field."
Basically, that means a magnetic field would cause a tattoo to vibrate in a specific way. The tattoos, the patent suggests, would have to use "ink enriched by ferromagnetic or paramagnetic compounds" — not too tricky, given that tattoos already contain metallic compounds. Then different magnetic fields could be used to provide different sensations, allowing the tatt wearer to discern between different callers or different kinds of alerts.
While the patent does suggest that similar technology could be rolled into something else wearable, like a badge, the obvious application is for something truly embedded. At least you can never lose a tattoo. [USPTO via Unwired View via Wall Street Journal ]