I've been messing around with JSON for some time, just pushing it out as text and it hasn't hurt anybody (that I know of), but I'd like to start doing things properly.
I have seen so many purported "standards" for the JSON content type:
For JSON text:
The MIME media type for JSON text is
application/json. The default encoding is UTF-8. (Source: RFC 4627).
For JSONP with callback:
Here are some blog posts that were mentioned in the comments that are relevant.