"The Dojo and Windows Live Platform teams have both recently released DHTML hacks that allow two iframes in different domains to communicate, bypassing the notorious same-domain policy implemented in browsers. I'm surprised by the relative lack of response in the AJAX blogosphere, as this opens lots of possibilities for mashups." (Curiousity is bliss)

This hack allows you to create a webservice factories located in a dedicated server that is simply responsible to manage mashup from external domain (which can including caching policy, etc).