open → assigned
DataType "jsonp" is not ignored at all. Ajax simply uses the best available transport (ie the one with the best error handling capabilities).
The case where you request a same-domain url that redirects to a cross-domain one shouldn't force all other valid usages of same-domain script ot jsonp requests to use script tags.
Since the rewrite there's an internally computed option called "crossDomain". I think the best way to deal with this kind of situation is to only compute crossDomain if no truish value has already been provided for it in the options. That way, you can force a same-domain request to be considered cross-domain (and handle cross-domain redirection) while not forcing a less reliable transport on all other same-domain requests.