This document discusses cross-domain mashups using jQuery and Google App Engine. It describes common techniques for dealing with the same-origin policy, including proxies, JSONP, and building sample applications that mashup Twitter data, geotagged tweets, and maps. Examples include parsing RSS feeds from Twitter into JSONP and displaying tweets on a map based on their geotagged locations. The document concludes by noting issues with trust, failures, and limitations for enterprise use.