The AP Stylebook czars put out a notice today that included an entry on disc vs disk.

disc, disk Use the disc spelling for phonograph records and related terms (disc jockey), optical and laser-based devices (a Blu-ray disc) and for disc brake. Use disk for computer-related references and medical references, such as a slipped disk.

This is an updated entry in the online version of the venerable style guide. There you have it, the definitive answer in disc vs disk..