nearly any plant that is wind pollinated can release its genes into the environment. the toxin-mechanisms introduced into food plants are only toxic to a specific plant pest (for instance, BtCorn produces a substance that will kill the corn borer pest - such a toxin would only be harmful to the pest you are trying to kill and not to humans). and yes, once released it is nearly impossible to stop the spread of the genetic material if the plant is wind-pollinated.
if there are any other questions/horror stories i can clarify about this issue dont hesitate to ask me.