Well since NV couldn't manage to put out dual GTX285's, and Fermi will use more power than a GTX285, anything they put out will be very cut down dual Fermi, so even if Fermi outperforms a single HD5870, a two GPU product might not exceed the HD5970.
And dual Fermi probably won't be seen particularly soon either, so there's no real need for ATI to put out a single GPU card which is loads faster.
At the end of the day, when Fermi launches, ATI will have the fastest single card in most situations, whether or not "Trillian" is a major step up from the HD5870. (HD5970 being a single card, since it uses one PCIe connector).
When it comes to dual GPU vs dual GPU, if there is a dual from NV, they will both be at the limits of power consumption, and it'll be a case of who can make their card most power efficient, not who has the most raw potential.
AMD/ATI have won now, they will be winning when Fermi launches, and the competition between fastest dual GPU card won't be one of raw potential, but power efficiency.
NV can't blow ATI away, because it's not possible unless they ignore the PCIe specification, therefore ATI have no true reason to be afraid.