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I would say this is the first stage in the emergence of a new species. After many more generations of small headed snakes breeding, sooner or later they will either die, or the environment will change so that the big headed snakes are no longer being selected against, or they will be unable to breed with the big headed variety and a new species will emerge.

It's like that butterfly (or moth) that responded to bad air pollution in England and became darker so it couldn't be seen by predators. On a white tree, it was conspicuous, but on a tree in a polluted area it wasn't. The population became almost all dark. However, GB cleaned up its act and its air and the population has shifted back to the earlier white variety, since the trees are now cleaner. There wasn't enough time and selection for the dark variety to emerge as a new species.

If the Aussies start eating the toads, the big headed snake may reemerge as the dominant variety. If not it may disappear to be replaced by the small headed one. You can let that nutcase Steve Irwin decide if its a new species or not.

There's some doubt about the peppered moth example, but, yes, it's possible that after many more generations, the small-headed snakes will no longer be able to interbreed successfully with the large-headed ones (if any large-headed ones are still around). Perhaps, too, there will be a few hardy large-headed snake individuals born with a slight immunity to the poisonous effects of cane toads (random variation), and which, after ingesting only a baby cane toad, 'learn' not to eat cane toads, successfully breed, and so begin the process of developing a large-headed snake population that is immune to cane toad poisoning. Of course, that would probably take many, many generations.

Insufficient appreciation of the cumulative effects of small changes from generation to generation seems to me to be one of the characteristics shared by many who adamantly oppose evolution. It's not as if nature doesn't offer us many examples of such cumulative effects at work. Take a little water flow along a sloping plain, add several hundreds of millions of years, and, voilá, you get a Grand Canyon. Etc.

The living world is (to borrow from Keats) "the foster child of silence and slow time", with the emphasis on "slow".