Will the nano have a smaller screen? If so, it's less likely. Or at least it's going to have reduced features. Internet on a smaller touch screen won't cut it.
There's a lower limit size in which the finger gets too big in relation to a small touch screen for example. Yes, you could get an Apple nano-iPhone, but i'd think they'd rather have a higher-density screen, or at least going OLED.
at some point push resolution independence perhaps?
If they bring out a tablet, then they could nudge developers to think about the problem. Would be an interesting addition to the App Store also, if they started using the iTunes App Store for other apps, in a kind of Valve's Steam like way. Side tracked.