There's no specific law, but then there wasn't for using a phone and people were still done for it before the specific one came out.
Driving without due care & attention is what they've have you for if you didn't have an accident, just like eating, drinking, trying to find a CD or anything that causes you to not be concentrating on the road. This assumes you're using it with the earphones btw.
Whether you'd actually get taken up on this depends on the officer you met, but some are a right stickler for the law, so best use an iPod FM transmitter.
If you want an example of how annoying they can be - someone was prosecuted a couple of years ago for not having their hands on the wheel, even though they were standing at a red light! Got 3 points for not keeping the car under full control. You really want to make their day?