I'm not all that familiar with XMLHttpRequest, but every JavaScript example I can find sets the response type between open() and send(). I don't understand John's concern about a race condition since it seems like the request won't be sent until send() is called? I think we should follow common practice.
Since this API is essentially just a wrapper around the JavaScript object and there's no place to stash the desired response type, adding the setter and deprecating create() with a response type seems like the way to go. https://gwt-code-reviews.appspot.com/1830803/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
