ORM for Javascript/JSON
I may be missing something here, but JSON (JavaScript Object Notation) is already a Javascript object in and of itself.
If the data you are getting from the server doesn't map well to a usable Javascript object, I would argue that it's the server side that needs to change to return a more useful serialized object rather than a simple recordset.
There is CouchDB which is a DB written in Erlang that uses HTTP as the transport. This eliminates the need for middle-ware and permits you to navigate the DB directly with AJAX calls. I can't speak good or bad about it. I haven't heard much about it in months and it seems as if the hype train departed a few years ago.
You can't have an ORM to a remote DB in Javascript.. An, ORM requires transcendental knowledge of the DB schema, and sending that out with an API just isn't that pragmatic as of yet.
For persistent local storage, there is the now deprecated Google Gears and the HTML5 Clientside DB.