The ATI Radeon HD 4000 series apparently has Nvidia concerned. Concerned enough to consider dropping prices, and the latest rumors are that Nvidia has a GPU price cut planned sometime in the near future. AMD's significant gain in GPU sales, upwards of 40% for the Q3, has no doubt prompted this action, and Nvidia seeks to prevent themselves from losing any more market share in the lucrative Q4.
Even though the 4000 series has performed admirably and definitely given Nvidia a scare, the green team is by no means swimming in the shallow end of the performance pool. Both Nvidia's and AMD's high end cards are neck and neck, so without a sudden technical advantage, a price cut is the most logical way to start picking up more ground.
Price cuts are expected for some of the 9000 series cards as well as some of the 200 series cards. If this is true, it would be better for Nvidia to implement them before the end of the month.