How does the protocol (transport or security) conversion happen? Is the developer responsible for it or does the mashup platform take care of it? What i meant was when we talk about using the same mashup on different platforms/channels.

The Mashup platform takes care of protocol. The mashup platform handles the formatting appropriate to each of those destinations/channels. So we put the mashup data out in SMS or as an iPhone app for mobile devices. for example. We produce a portlet or RSS feed for the portal or dashboard.