I think menthol is regulated by the FDA in America, so maybe a while before they get it worked out. Not sure its worth the expense to run an mp3 player 60 hours. The ipod mini is suppose to run 18 hours on a charge, plus you can get a battery pack to extend the run time.
I rather they develop this technology for power hungry laptops and flashlights. My asus laptop has a centrino chip and runs 9 hours per the 9 cell battery. Its fairly small and light as is.