This is a no-brainer. Don't leave your phone, iPod, or any other mobile device in the car especially during hot summer months. With global warming (alright, climate change for the conspiracy theorists), we can expect warmer weather for longer duration each year.
It stands to make sense that we shouldn't leave our digital companions in the car, direct sunlight, or extreme heat conditions. Yes, extreme heat conditions. I've seen folks take their music players into hot rooms and saunas.
According to reports, an iPod is suspected of lighting this car on fire. Though police merely suspects the culprit, they've yet to pin down conclusively it was an iPod. Though the damage in the picture was a bit much.
Regardless, it's good practice as far as keeping your mobile devices in good working condition if it doesn't have to deal with extreme conditions, including cold weather or a wet environment.
My suggestion is that you find out from your manufacturer or manual the range that is best to operate your device. It's not something we think about much but if you happen to be traveling, that might be important to know. For instance, iPhones were known to be rejected by Apple in the Singapore possibly due to the high humid conditions there.
And as always, practice safe battery use and conditioning. It'll go a long way in extending the battery life.
Plus, not leaving your laptop or mp3 player in the car is good for another reason. Theft.