The full paper for the new study by researchers at Cornell looking at seven years of Bt cotton cultivation in China is now available on the 'Bt cotton resources' page of GRAIN's website.
http://www.grain.org/research/btcotton.cfm?links

From the paper:

Tarnishing Silver Bullets: Bt Technology Adoption, Bounded Rationality and the Outbreak of Secondary Pest Infestations in China (Shenghui Wang, Jul-2006): "Seven years after the initial commercialization of Bt cotton in China, we show that total pesticide expenditure for Bt cotton farmers in China is nearly equal to that of their conventional counterparts, about $101 per hectare. Bt farmers in 2004 on the average, have to spray pesticide 18.22 times, which are more than 3 times higher compared with 6 times pesticide spray in 1999. Detailed information on pesticide expenditures reveals that, though Bt farmers saved 46% Bollworm pesticide relative to non-Bt farmers, they spend 40% more on pesticides designed to kill an emerging secondary pest. These secondary pests (one example is Mirid) was rarely found in the field prior to the adoption of Bt cotton, presumably kept in check by bollworm populations and regular pesticide spraying. The extra expenditure needed to control secondary pests nearly offsets the savings on primary pesticide frequently cited in the current literature."

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