Nissan isn't doing well because they are not responding to the market fast enough. They sell a third of the cars that Toyota does and half of what Honda did.
Nissan offers no hybrids and those vehicles consistently bring people's attention to your brand. Combine that with a lot of smart car buyers knowing that 07 brings some real changes to your line and you have to work extra hard to sell cars.
Infiniti is in the same boat. They are ignoring the hybrid move claiming they are a performance brand yet Lexus proved you can make hybrids that perform. Of course the quality level at Lexus is worlds apart better than Infiniti and Lexus recently introduce new Is perform as well if not better than similarly priced Infinitis. Lexus sells 3 times the cars as Infiniti, same as the Nissan to Toyota difference.
All of this means that is if you want a good deal on a Nissan or Infiniti you should be able to leverage their falling sales into your favor. Especially at Infiniti, my local dealer is one of those list price types, but a nearby dealer is now allowing to equip, price, and even submit a "bid" on new Infinitis. They show both MSRP and Invoice prices (not the true invoice price - the type that KBB/Edmunds show). Thats a good step.