Cases purported to be for the rumored fourth-generation iPod nano and second-generation iPod touch have appeared on Alibaba (here, here and here). Made from silicon, and seemingly tallying with the leaked image we've seen recently, the design for the nano suggests Apple would be going back to its earlier, long & thin form-factor.
As for the touch case, it seems to include a cut-out notch for the light sensor (which dims or brightens the screen backlight depending on ambient conditions). Of course, you should take all of this with a huge pinch of salt; there's no guarantee that these aren't cases for PMP lookalikes or iPod clones.
Alibaba also features what claims to be a plastic iPod Touch case, but closer examination suggests that it's actually intended for the iPhone. The broader cut-out across the bottom (to accommodate the speakers) and the cut-outs on top suggest it's intended for the cellphone not the PMP.