Interesting... are you located near any towers, or in a city? It's possible that the length of the headphone cable is just right for picking up a local station, or that the actual headphone amp inside the ibook is picking it up as it's capacitors discharge. Inductive reception.
At the theatre where I work, there's a radio station about a half mile away. When equipment is off (or sometimes when its on, unfortunately) we can hear the station. Very common through keyboard and bass amplifiers.... and our wireless mic system.
The oddest was when i was working for a security systems company, and there was this family that was hearing people talking through their household intercom late at night. Turns out that a cell tower had gone up about a block away, and it would bleed through around 2am. Cell phone conversations house-wide. Drove them nuts until i installed some ferrite beads around the interconnecting wires, which were acting like huge antennas. Solved the prob.
