What's the most appropriate HTTP status code for an "item not found" error page

Getting overly clever with obscure-er HTTP error codes is a bad idea. Browsers sometimes react in unhelpful ways that obfuscate the situation. Stick with 404.

A 404 return code actually means 'resource not found', and applies to any entity for which a request was made but not satisfied. So it works equally-well for pages, subsections of pages, and any item that exists on the page which has a specific request to be rendered.

So 404 is the right code to use in this scenario. Note that it doesn't apply to 'server not found', which is a different situation in which a request was issued but not answered at all, as opposed to answered but without the resource requested.

404 is just fine. HTTP/1.1 Status Code Definitions from RFC2616