Dr. David Kerns has published this excellent seminar (spoken plus powerpoint) on the current status of Bt technologies in cotton. He goes into great detail on the history and developement of Bt toxins, how they work and are expressed in the plant, and the development of resistance in target insects. The real focus if the talk is an analysis of the costs and benefits to spraying Bt crops, as well as related thresholds. In times of low prices, saving a few dollars an acre by preventing a pyrethroid spray, which would also disrupt beneficials could be vital.
The seminar is less than an hour long and packed with trial data that you may find interesting.