A note on the new iPod Nano battery life: I've had mine last up to seven hours uncharged, turned off roughly 30 minutes of those seven hours. It was in my pocket the whole time during an outdoor session of a skatepark. That being said, songs were not skipped, the volume was not altered and the temperature was roughly 75 degrees fahrenheit outside and then however hot and humid it was in my pocket.
I know people who have a variety of other MP3 flash players and have had no issues with them what so ever. If you end up having a question on a specific model (or perhaps I can list the ones more contactable friends have) and want feed back on them, I can get it for you. All of us BMX in our free time, so the conditions are a little harsher than what the average jogger puts on a portable music player.