Although not completely off the rumor mill yet, it appears Nvidia may have canceled the 7800 Ultra
in an effort to keep their two flagship cards, the 7800 GT and 7800 GTX, in the limelight. The primary reasons cited are that the GTX can be and is already being released in volume, and that with ATI's continuing supply problems Nvidia has no reason to out-do their own product already. The 7800 GTX is already an extremely expensive, very powerful enthusiast level card, and the article notes it may upset people to have such a pricey card be outshined, and continuing with Ultra could possibly hurt sales of the GTX.
ATI has promised to not have the supply issues faced last year and earlier this year (especially with the X700), but time will tell.