This is from the book "The Perfect Thing" by Steven Levy:
"Once as I began an interview with him [Steve Jobs], I took out my microphone-equipped iPod to record the conversation. The device was covered by a translucent plastic iSkin cover. He looked at me as if I'd smeared the Mona Lisa with cow excrement. I tried to explain that I wanted to prevent the smears and dings that iPods often attract. He wasn't having any. "I think the stainless steel looks beautiful when it wears," he said. "Probably it's like us. I mean, I'm going to be fifty next year, so I'm like a scratched-up iPod myself.""
I don't think he uses a case, although I also like the theory that Steve has an unscratchable iPhone.