Much has been written recently about hearing damage caused by listening to the iPod (and other DAP players) at high volume. A little common sense will ensure that your hearing is not subjected to loud and damaging levels of music. For advice on protecting your hearing, read the iLounge Article Listen Safely: Your Ears and Your iPod. This informative article also includes links for further information.
Apple have released iPod Updater 2006-03-23 which includes a volume limit. For information on setting this limit Apple have published an article Setting the maximum volume limit on iPod.