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Um, so? Why exactly would you be using IE to retrieve JSON? It's an API, right?

There's all kinds of client side uses for APIs. Facebook Connect logins for instance, which can't be initiated server side.

Sending application/json to older versions of IE can cause it to pop up Save As dialogs, even in AJAX requests.

They did the same thing with application/xml+xhtml.

Presumably as part of an AJAX request?