Attribute II sweet corn expresses the vegetative insecticidal protein Vip3A and the crystalline protein Cry1Ab, both of which are derived from the naturally occurring soil microorganism Bacillus thuringiensis (Bt). Bt spores and proteins have been used worldwide for many decades in both organic and conventional farming as a direct application pesticide for vegetable and fruit crops, and thus there is a long history of safe use and exposure. Both of these proteins are highly selective in terms of their toxicity because they bind to specific receptors that are found in the midgut of certain insect species, but not found in other insects or vertebrate animals. These two proteins are therefore only toxic to the larvae (caterpillars) of certain Lepidopteran insects that are key agricultural pests, but are harmless to other insects, fish, reptiles, birds and mammals, including humans. Here is a recent article that discusses the safety of Bt proteins that you may find useful:
Hammond B.G. and Koch M.S. (2012). A Review of the Food Safety of Bt Crops, In: Bacillus thuringiensis Biotechnology, E. Sansinenea (ed.). Springer: New York. Pp. 305-325.
To answer your second question, long term studies have not been conducted of Attribute II, nor are they necessary. Attribute II expresses transgenic proteins which have been demonstrated to be: 1) rapidly digestible by humans as other dietary proteins, 2) denatured and inactivated by heat (as from cooking), and 3) completely nontoxic in laboratory mice and rats exposed to levels many hundreds of times higher than humans could possibly be exposed to. As these proteins are easily digested and nontoxic, and as Attribute II sweet corn has been demonstrated to otherwise be substantially equivalent to conventional varieties of sweet corn, there is simply no need for further testing as there is no biological plausibility for chronic toxicity potential. Here is a recent article that supports this position that long term studies are not necessary to evaluate the safety of genetically modified crops:
Snell C., Bernheim A., Bergé J.-B., Kuntz M., Pascal G., Paris A., Ricroch A.E. (2012). Assessment of the health impact of GM plants in long-term and multigenerational animal feeding trials: a literature review. Food and Chemical Toxicology 50:1134-1148.
Lastly, expression of these proteins by Attribute II and other corn varieties is beneficial in terms of food safety. Minimized insect feeding damage is directly correlated with reduced infection of the corn ear by the ubiquitous fungi that produce mycotoxins, many of which potent liver and kidney toxins and carcinogens.