Headphones can be dangerous because they can cause hearing loss.

Hearing Loss Display

By having headphones that predict the length of time that headphones can be at a high volume, it can limit the duration of loud sounds being played through the head phones to protect the listeners ears. This could be incorporated into an Ipod, considering the ipod OS know's what the volume is and how long it has been there. the ipod could flash with a warning and give the user a option to turn it down. It can be to annoying, but it might suggest something.

The hearing can be damaged by a single loud sound, so you cannot use a safe time limit at a certain very high volume. Use a limiter instead, which limits the sound level to safe levels at all times.

Directional Speakers

The speakers in the headphone can tilt away from the center of the ear drum as the volume is turned up. This can prevent hearing loss. This will work well in headphones that fully cover the ear, not the buds.

Surround Sound

Multiple little ear buds inside one Bigger headphone, to give a sence of direction of where the sound is comming from..