There was that strange HP iPod (
http://en.wikipedia.org/wiki/IPod%2BHP) which, to this day, weirds me out. (It's also ancient history at this point in time. Apple sold only one gadget back then and it didn't run iOS. It's astonishing how Apple changed in the last five years.) You never know what Apple might do but I think you are very correct when you say that Apple usually doesn't do co-branding.
Edit: Another point I just thought of and wanted to add: Neither Nike+ nor the HP iPod are or were ever mission critical for Apple. Nike+ is nice to have but if it disappeared from the face of the Earth tomorrow Apple wouldn't really be any worse off. Co-branding is apperantly something Apple does when the products have already become stupendously succesful. (Nike+ was added to iPods at their peak.)