There's no specific law against it in this country* (can you imagine the uproar if they brought in a law of something you can do in a car but not on a bike!) But as said, you have to consider if it would directly lead to a charge of Driving without due care or somesuch...
That is the only justification for claims that you may have heard it's illegal.
*I have heard a lot of reports that in France the use of GPS with a headset on a bike is illegal
(but unconfirmed)