Knocking on deaf's door, The Who guitarist Pete Townshend posted a message on his website warning of the dangers of cranking the volume on your iPod. That's a really valid concern and I have a similar warning.
After you're done listening to your iPod in your car, turn the volume down. When mine is hooked up to my stereo (via cassette adapter), I have to max the volume on the iPod to hear anything. Last night I forgot to turn it down after I came home and today I went to listen to it through headphones and - BAM - I nearly blew my eardrums out. Well, it wasn't that severe, but it didn't tickle.
So, there you go, kids.