There shouldn't be any problem power wise, but I stopped leaving my iPod Classic in the car because it completely failed when I left work to go home on the second day I did this. The high that day was only 76 degrees. It was working at lunch, so my best guess is that the hard drive overheated during the afternoon. It couldn't do anything but power up; I turned it off immediately! My thinking was that the hard drive "warped" and couldn't spin. I figured that it would be OK when it cooled down, so about 9pm I went out and it was working. The battery was completely drained, however, and it was behaving very oddly as a result. It was fully charged before it stopped working, that's why I thought there was some mechanical problem with the drive putting an additional load on the battery! Once I charged the battery, there seemed to be no ill effects!
The risks to iPods and iPhones with solid state memory is much less, but they also warn against being left in hot environments (like inside parked cars). I solved this problem by starting a mp3 memory stick project. It's more work than an iPod, but it's been in the car for weeks now and working flawlessly. It's also very small and I can close the console access door over it, so no one knows that it's there. No temptation to would be thieves!
