I can't tell you exactly what this wire is for but the picture shows an "M5" marked daughterboard so I guess this is an X5V (without the radio)? This could possibly be an important piece of information for someone who wants to answer.
And I think it could be something in relation to the remote control because this is near the remote control socket (it would be below the headphone jack you can see there in the top right corner of this picture. Did you notice any problems with your device that could be hardware related (maybe with the harddrive or audio) or do you just ask because you don't know if this should be the way it is?