Today, AppleInsider is pointing to fully illustrated take-aparts at iFixit.com - one for the iPod nano, and one for the classic. They note that Apple abandoned the anodized aluminum enclosure of the former nano for the scratch-hungry gloss of the original. Also, Apple used lots of adhesive with the nano, which makes getting it back together difficult.
As for the classic, it has metal enclosure tabs instead of the old plastic ones, making it harder to get apart.
If you're still compelled to disassemble your iPod, iFixit has the how-to.