How does this compare in construct and operation to the Portal/Portlet style of web page composition.

Much of Presto’s mashup creation/sharing functionality can be driven by non-developer types of users.  This is traditionally not the case in portals/portlets.  In addition, mashups are capable of delivering their information to a much wider variety of destinations, including (but not limited to) portals, desktops, spreadsheets, web pages, mobile devices, and standards-based data API (like RSS/ATOM, REST, SOAP).