Here's Quiver's: https://github.com/HappenApps/Quiver/wiki/Quiver-Data-Format
It's . . . really good. Like, _really_ good.
I particularly like that they keep the metadata and the content of the note separate. This is the same thing Camlistore does, and it's nice for version control because making changes to the metadata (adding new tags or whatever) doesn't show up as a content change.
Speaking of version control, Quiver's thoughts on this are interesting:
"Since Quiver stores all the data in plain JSON files, it’s easy to put the whole library (or a specific notebook) under version control. This is another way to collaborate with your team."
Leaving version control up to the user isn't the worst, but it does mean that it won't be straightforward to put an "undo" in the GUI (if the GUI already has undo somehow I'd like to hear how they do it).
I'm also not sure whether it would be better to version the whole notebook or individual qvnotes. I have more thoughts on this (starting from a slightly different angle of how it relates to wikis) here: http://housejeffries.com/page/4.
EDIT: Something just occurred to me. If HappenApps wanted to make a machine readable schema for qvnotebook and qvnote they could use JSON Schema for the JSON part. What would they use for the directory part? Is there such a thing as a schema for directories? (e.g. this directory must contain a `content.json` file, a `meta.json` file, etc.)