This is precisely why I think Apple would want the nano to use the OS. It's very expensive to develop an OS, and it makes little practical sense for Apple to make a new product only to have to develop a comparable operating system. "Oh but the nano iPod uses a different OS." Right... but that one was developed before the iPhone/iPod touch OS existed, and so of course it will still be used where appropriate. But why would Apple make a small device, mostly useless, and very much like every other regular phone, that requires a new OS? Seriously, why?
It's pretty much just a given that if the iPhone Nano comes out it will use the same OS, but that doesn't make it a smart phone just like the iPod Touch using the iPhone OS doesn't make it a phone. All they would have to do is remove the smart-phone apps. The OS doesn't make it a smart phone, it makes it an iPhone. On a related note, an iPhone Nano would not be just like the current iPhone without a required data plan. The current iPhone is quite capable of being used without a data plan; it's not specific to the device, but rather AT&T'a policy becuase the iPhone is classified as a smart-phone.