Guardian goes up the pan slightly, I received a reply:
Dear Platinumsage, thank you for your email.
"Nice" is the correct way of referring to the body, as per our style guide, which advises that only the initial letter is capitalised for acronyms (abbreviations that themselves are spoken as words - eg Nasa, Unicef, Rada).
However, I think our editors should use common sense in this specific case, and Nice should not be used at the beginning of a headline or subheading, as these are the only places where the acronym could be confused with the word "nice", devoid of further context provided by the body text.
I passed this query on to the relevant production editor, who arranged for it to be changed.