As i said, place a G5 next to the new iMac and your instantly struck by how outdated and unrefined the earlier model is - despite it being awesome when it was released. For those that don't own one, just google the G5 and you'll be surprised by how ungainly it now looks. Subtle changes like the chin etc. have really enhanced the design. Same for the iPhone.
this is an interesting point but I'm not sure the size of the iPhone allows for changes in design akin to the G5 vs iMac. other than some of the small suggestions (different material, different backing), the phone really needs to DO certain things and not LOOK a certain way. The key to the phone is the fact that the device mostly disappears -- it's all about the software. I think all the various designs of the iPod suggest a change is quite possible, but personally i'm not convinced it will happen.