If a couple of new ideas on the personal media player front become mainstream, then it's possible that your next-gen MP3 player will be able to sense your mood. And, in a limited way, it'll empathize by choosing appropriate music.
The first idea comes in a recent patent by Sony Ericsson simply headed "Generating a playlist based on facial expression," and it does more or less exactly what you imagine. The technology, which includes a camera, would go into your smartphone media player. By taking a peek at your features and analyzing the imagery, the player would be able—in theory—to detect if you're smiling, angry, bored, or maybe sad. And then it would cue-up appropriate music in a playlist for you, possibly using tech like Sony's SenseMe that scans through your tracks and analyzes them for details on the musical feeling of each.
Basically it's an enhancement of the kind of instant image analysis we're getting used to in our compact cameras, which can now detect faces, and then spot if anyone's eyes are shut or they're not smiling enough.
The second idea has a bit more solid thinking behind it, largely because it's simpler. It's a design project by Anaid Gomez Ortigoza, an NYU student. Dubbed Kokoro, it basically senses your mood using your heart rate—detected via a built-in Polar heart sensor. Depending on how fast your heart is beating—which is a pretty good indicator if you're in the mood for something dramatic or mellow—it'll choose music with a similarly fast or slow tempo. And if it chooses slow jazz when you fancy some techno, then there are buttons to override, and vary the tempo it's chosen.
Sounds like both technologies could be amusing additions to media player functionality. Hopefully they'll be good at matching your mood. I have a feeling if—the Sony one in particular—chooses Stand by Your Man, or—worse—Cotton-eyed-Joe while you're flaming angry after receiving a parking ticket—it would probably make you more angry.