you hear much talk about the iphone's battery life. well, here's my hint.
put the thing in airplane mode, turn off wi-fi, switch it to auto-brightness, and use it like an ipod except when you explicitly intend to make or receive a call, email, web surf, etc.
it's said a battery charge will get you 24hrs pure audio. i think more, actually.
so far i've got 17.5hrs+ of such use and my battery is showing just under 50% – making me think careful iphone use could get you 30hrs+ between charges if you play it smart.