Jeff, Disco Dick Indeed!
As stated, the use of such devices is legal as long as nothing is listened to which could aid the player, for example, a song which helps with tempo is illegal. Two other notes.
1. Many organizations prohibit the use of electronic devices, such as cell phones and PDAs, and may include iPods and similar in that prohibition.
2. The ruling bodies are carefully studying this whole area and may ban the use of all such devices in the future.