Interesting topic, and telling banter. All I really have to add is this...
Some of the very worst things man has done have been done with the best of intentions.
I think of the plants that are taking over much of Hawaii's natural flora, the previously mentioned multiple and massive critter invasions in Oz and NZ, and even the US is now dealing with a recent EPA/USDA introduction of Asian Beetles, which look like orange Ladybugs/Lady beetles but stink and bite and have almost completely wipes out Ladybugs where I live. They also have populated in much larger numbers than their rivals, simply covering entire buildings, etc in parts of the Midwest in the fall. I simply can't believe this was the best way to control aphids...
All these things were all introduced intentionally for hunting, collection, or to erradicate a previous mistake (ie: the ferret problem). Good intentions, all, but here is the key...
A lack of complete understanding of an entire ecosystem will necessarilly cause an oversight in any attempt to modify it in a controlled fashion.
Genetic engineering is no different; except that the undesired results could be much more drastic than in more natural practices, and they happen much more quickly than with selective breeding over generations...You can get into more trouble much more quickly, if things don't go as planned.