Let's be clear: There has been speculation about what might be there in its place, but whatever it is, it definitely isn't iOS.
iOS, like Mac OS X, contains BSD-licensed code derived from FreeBSD. As a legal consequence of that code being incorporated, Apple must provide a copyright notice advising users of the fact that FreeBSD-derived code is present in the software. This notice is included in the About screen or other accompanying documentation of every iPod touch, iPhone, (presumably iPad, but I've never used one in person to verify this), and Mac OS X powered PC. It is not included in the About screen, or any other accompanying documentation, of the current generation iPod nano. Therefore, it cannot be running FreeBSD-derived code, so it cannot be running iOS.
Reverse engineers have also pored over the firmware upgrade files served up by Apple, and they have also reached the conclusion that they don't recognize exactly what it is, but at the same time, they do know what it isn't.
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