Odd, I've never heard it in the plural and only ever read it in the plural in old historical documents. Every prescriptive grammar I've seen also has it in singular. Where are you from? Might be a dialectical thing. Like how the English use the plural for collective nouns like "government" whereas we'd only ever use singular in the US.
I"m Canadian. Like I said earlier post, I don't dispute that the "United States" can be, or even may generally be, treated as a singular noun. This is why I didn't really push the issue when I first raised it 5 months ago. That said, when presented with the bare sentence "Gdzie są Stany Zjednoczone", with absolutely no other context, "are" strikes me as more natural.
Perhaps it is a "mistake", perhaps it is dialectical, or perhaps even I am simply being miscued by the "są". My feeling, however, is that that "are" should be accepted as an answer (as I understand from Marek's post that it now is), given that the "mistake" is an English issue, not a Polish one, and that a learner making the "mistake" is not all evidencing a misunderstanding of the Polish sentence.
It should be singular in this sentence, because "the United States" is the name of a country, singular. It's the same with the United Nations: "Where is the United Nations?" is correct, because the UN is one thing. To get a plural, you'd have to ask "Where are the nations of the United Nations?" and "Where are the states of the United States?"