Back in 1997, I was experimenting "virtual apparatus framework" in order to allow "components" on a page to communicate among themselves. [see also]. I faced great technical difficulties and unstable "live connect" implementations in various browsers.
10 years later, Google is calling "virtual apparatus" iGoogle gadgets and they are trying to enable these gadgets to publish and listen (one-way communication) and only limited from type="html" and type="html-inline" gadgets. I don't know what that means because I did not have time to really look into iGoogle gadgets.
An obvious hurdle will be allowing gadgets hosted from different domains to communicate - that "cross domain scripting" security model!