Nvidia might not hit the mainstream or low end market with their high power fermis.
A lot of people can't take a GPU that takes up 200W and more. Even if the low end runs cooler and at a lower voltage they require AMPs like no tomorrow. Most people only have power supplies with 25A rails.
That's what I think also considering the extra cost of the cooling that those higher amps require as well-- besides the fact that the cost of the Fermi chip places it a bit high for use in a lower end product and with the 480 and 470 already being partially non functional chips (not using the entire original design of the chip) they probably are using most of the functional dies for the production of them already and do not have a big surplus of chips that could be used for a lower end product to make it a feasible use for the partial defect chips from production.