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Coincidently I stumbled into this ANZAC/Anzac difficulty all on my own today when I e-mailed an American before even being aware of this thread. (I've been away from home for two weeks).
In the letter I mentioned Anzac Day. As I was writing to somebody who had most likely never heard of it I explained that it was like their Veteran's Day. I then went on to give a break down of the title i.e. (ANZAC - Australian & New Zealand Army Corp). Having set out what the title stood for I felt compelled to go back and change the Anzac Day to ANZAC Day as I had made it clear that it was an acronym.
In common, spoken usage, Anzac is a word and nobody in conversation would dream of spelling it out. It may defy convention but IMHO it probably depends as much who you are writing to as to whether you use the word or the acronym.
Maybe to save arguments and avoid offending anyone we could use 'anzac'. That's funny, my spell check just tagged that and suggested Anzac?
Frankly, my dear IDGAD. Or is that Idgad? Either way, none of this is worth getting upset about.

David