The strain of corn being cited probably accounts for more than 60% of the corn grown in the US, and possibly 40% or more of the corn grown worldwide.
Just wanted to clarify a point here: bt11 is not a "strain of corn" any more than OnStar is a model of car. The bt11 gene is integrated into thousands of varieties of corn, from tropical sweet corns to Canadian flints, just as the OnStar system is integrated into many different models and makes of vehicles. These varieties of corn are developed first, without the bt11 gene, and then the gene is later bred into the line in what most companies call the trait conversion process just as base trim levels of cars are developed, and then add-on systems like OnStar and stereo systems are tacked on later as features and options.
Any type of monoculture weakness that could have the kind of impact you're describing would have to be a result of the bt11 gene itself. This kind of thing has happened before in corn, though no case of that has (so far) been associated with GM traits (e.g., T-cytoplasm, which was used to facilitate hybrid production in many different varieties caused susceptibility to gray leaf spot).
In car analogy terms, what you're afraid of would be like the OnStar system itself causing engine problems in every type of vehicle with which it is integrated due to a flaw in the remote engine ignition feature. This is in contrast to a true monoculture situation, which would be like if everyone drove a ford pinto and we suddenly found our highways clogged with burnt exploded wrecks every time an accident occurred. The first situation is easy to fix--just buy the base model instead--whereas the second is trickier if all every manufacturer makes and knows how to design anymore is a pinto.