So someone leaked the GH III demo onto the internet. You can download it here. http://www.xboxlivetheguide.co.uk/News/phpBB2/viewtopic.php?t=1653&postdays=0&postorder=asc&start=30
All you have to do is burn the .iso to a cd or dvd and you're good to go. It actually does work. Theres 5 songs on the demo and its pretty awesome. The battle mode is super fun.