I had this kind of idea too, but I think it should be the current iPod classic. They shouldn't make it into a massive publicity stunt - just quietly add the sig to the next 10,000 classics sold?
I think this a great idea, but I would release the current iPod Classic - exclusively in white with Steve's signature engraved on the back. $349 - but $150 goes to Pancreatic Cancer Action Network (Pancan.org).
I imagine that Apple could sell a million without too much effort.