According to the article, NVIDIA believes that rival manufacturer ATI will not be able to ship a great number of R520 XT running at 600MHz/1400MHz, at least before December. The G70 Ultra was supposed to be a faster version of the G70 GTX card, both based on the existing chip, with the exception that it was just clocked faster. The main reason forcing Canadian ATI to delay the launch of its competitive R520 GT are probably the issues the company has faced during the implementation of a 90nm manufacturing process. By canceling the G70 Ultra, NVIDIA could take advantage of ATI's delay and sell more of its highly available 7800GTX's.
Basically, both the GTX and Ultra versions of NVIDIA's 7800 series share the same chip but different clock. However, hardware benchmarks have proved that there is no huge performance increase when running this chip at 500/1400 MHz with a fast (4000MHz+) CPU. At these speeds, you need to set ultra high features with FSAA and Anisotropic filtering to the maximum at resolutions such as 1600x1200 in order to show the difference.
ATI has not commented on the news. Its R520 GT is expected in the market at Christmas, and until then, only speculations can be expressed about the performance of the card compared with NVIDIA's solutions.