Last year I made a service that converts data from spreadsheets or spreadsheet-like text files to XML. http://elev.at/
How do you call methods other than GET? Maybe you should add a method parameter, since JSONP is GET-only.
I only currently need the read-only APIs, but adding other methods shouldn't be too difficult.
Ok, just added support for other methods, although I haven't had the chance to properly test it yet. Just send a POST/PUT/DELETE, with the url parameter and the rest of the parameters will be passed on - probably won't work with a multipart post.