http://ajaxian.com/archives/jsonrequest-proposal
I read the proposal a little, and found that it is eliminating cookie information in its request header.
http://www.json.org/JSONRequest.html
JSONRequest does not send or receive cookies or passwords in HTTP headers. This avoids false authorization situations. Knowing the name of a site does not grant the ability to use its browser credentials.
By ignoring cookies, this new crossdomain request approach might become safe, but on the other hand it throws aside a chance to enable personalized mashup. For example, if you wish to embed your e-mail inbox list in your favorite portal service, JSONRequest can't do that because they won't carry any cookies.
I think it might be better to have opt-in mechanisms for both remote services and end users. When considering that almost all ajax applications are now regarding JSON as only in the same domain, so opt-outing (like referer-based JSONP access control; this technique is used here!) is rather unsafe in this case.
Anyway, this is a new proposal, it may take some years to be embedded in almost all our browsers. I hope this opinion could reach him (or some communities) in some way or other.