@FumbleFingers - Let me agree that "ID" is short form of "Identity Document". But, we find a commonplace thing, such as, "What is your email ID" or "What is your email id". Obviously, the speaker is asking for "email Identity" or "email Identifier" and not for "email Identity Document" or email Identity Card".
It's actually about whether Id is short for "Identity" or "Identity Document", and therefore it's really about English. But it probably doesn't matter either way in code, since most camel-cased languages idiomatically only capitalise the first letter of acronyms.
Both the NOAD and the OED report that ID is an abbreviation for identity, identification. They weren't carrying any ID. I lost my ID card. The term id is used in psychoanalysis, and Id is a variant spelling of Eid. In some contexts, id could be understood as ID, for example in the phrase the user id used when talking of a CMS.
