Can Down's Syndrome people be classed as a different species of human?

That is actually a very insightful question and non something I've ever thought about. I understand you're thinking and don't believe there is a definitive answer to this question. As you've said Down's is due to a trisomy mutation. And thats the key, its a mutation. A 'species' of people with Down sydrome wouldn't naturally occur through evolution since a mutation is a 'spontanious change in the structure or arrangement of DNA'. Also if you think about it logically and applied a catch all mutation = different species where would we be! I certainly don't consider someone with albinoism a different species, nor someone with black skin. Also the definition of a species is 'two organisms that come together to produce viable/fertile offpspring'. I know my answer isn't definitive but I think I'll have to go with mutation, rather than speciation.