Here's my one thought for why I'd like to see an "iPhone Nano". I'm one of the few who thinks AT&T rate plans are ridiculous and will not change carriers just for an iPhone. I don't know the details of AT&T's exclusivity agreement, but if Apple were to introduce a separate, different model that doesn't fall under that specific agreement, they could make it an EVDO / CDMA phone for Verizon / Sprint networks-- further market penetration for Apple.
My sentiments exactly. If this product is real, who ever said its for AT&T? Since its technically a new product, new negotiations and deals with a carrier would have to be made. It may go to Sprint or Verizon (I think Verizon would be more likely). That, and just like the iPhone, the iPhone Nano may not be seen until the June timeframe because of prep.