You may be right partially in that Fermi may cause that slot to be filled. If Fermi is really good and priced competitively with ATI, ATI will probably lower the price of their cards. The 5850 may become priced at what a 5830 was going to be priced at and the 5870 would move down to the 5850 price point. That would be quite awesome.
It really depends on how Nvidia prices 360 GTX and 380 GTX.
If demand is still high enough to fully support both companies we may not see much movement in the way of prices.
If, However, Nvidia does start taking sales away from ATI then I would expect price drops on HD5870/50.
But seriously, what ATI cards would a Nvidia 360 GTX really compete against? Some people have speculated it that the low end Fermi may even be faster than HD5870.
The market is pretty small at the top end, it is not like Nvidia is invading with a high volume of mainstream chips. This is what makes me skeptical of large price drops.