Historically, browsers have been restricted to making XMLHttpRequests (XHRs) to the same origin (domain) as the web page making the request. However, the latest browsers now support cross-domain requests by implementing the Access Control spec from the W3C:
In order to keep older servers safe that assume browsers only do same-domain requests, the Access Control spec requires the server to opt-in to allow cross domain requests by the use of special HTTP headers and supporting some "pre-flight" HTTP calls.
Why should CouchDB support this: in larger, high traffic site, it is common to serve the static UI files from a separate, differently scaled server complex than the data access/API server layer. Also, there are some API services that are meant to be centrally hosted, but allow API consumers to use the API from different domains. In these cases, the UI in the browser would need to do cross domain requests to access CouchDB servers that act as the API/data access server layer.
JSONP is not enough in these cases since it is limited to GET requests, so no POSTing or PUTing of documents.
Some information from Firefox's perspective (functionality available as of Firefox 3.5):
And information on Safari/Webkit (functionality in latest WebKit and Safari 4):
IE 8 also uses the Access Control spec, but the requests have to go through their XDomainRequest object (XDR):
and I thought IE8 only allowed GET or POST requests through their XDR.
But as far as CouchDB is concerned, implementing the Access Control headers should be enough, and hopefully IE 9 will allow normal xdomain requests via XHR.