The USGA's ruling:
"The Rule prohibits a player, during a stipulated round, from using any artificial device or unusual equipment, or using any equipment in an abnormal manner, that 'might assist him in making a stroke or in his play.' Because a compass is not a usual piece of equipment in golf, and Bryson clearly stated that he had used the device to assist him, the USGA, R&A and the PGA Tour agreed it was in the best interest of the game to share this determination with Bryson immediately."
By this logic I question whether carrying a yardage book is legal. ☺