I am not a person to have bad hygiene, but once in a while I forget to cut my finger nails, never for too long, but usually i notice that i should have cut them a few days or maybe a week before i get around to it.
But with my new 15 inch powerbook, I always know when its time to clip them, because it tells me by being very annoying to type on if your nails are just a bit too long.
i actually appreciate this accidental feature, but I bet it drives almost every female on this drive crazy! (but then again normal keyboards also interfere with long nails, and women over time learn to ignore it)