The headphone jack is for audio out rather than audio in.
Ok, now that I have a mp3 player I'd like to play it on a boombox that I have. The boombox does not have a "aux in" jack but it has a "headphone" jack. I just bought a cable to use my mp3 in the car, which does have a "aux in" jack. Do I need a different cable for the boombox? Is it ok to plug the cable into the headphone spot on the boombox and then the headphone part on the mp3? Or do I need a totally different stereo with a "aux in" jack?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic