The new Bluetooth chip that’s in the 4S is very light on power consumption:
The phone, which went on sale Oct. 14, is the first to have a new type of Bluetooth chip that can connect using very little power. The chip uses so little power that it can go into devices that are powered only by a standard “button cell” battery common in watches. The battery can last for years.
Could be a great way to give the iPod nano Bluetooth connectivity without sacrificing battery life or form factor. And I can sure think of a few good reasons to put Bluetooth in the nano.
Update: I discovered that the low energy subset of Bluetooth 4.0 (BLE) does not have an audio profile. Sigh.