I like that I can carry my iPhone around in my pocket and it won't get scratched. Aluminum and glass are great for a phone.
BTW I recently had an iPod Touch in my hands and have to say it looks really good. The chrome back looks good, but is susceptible to scratches.
iPod mini (discontinued) and 2nd generation iPod nano: aluminium
As far as I know, every other iPod has had a chrome back.
trinemaria I like my iPhone too :D The iPod touch looks better though. It would be coolf they made the iPhone look like the iPod touch, but with an aluminium back like the iPhone has now.