The mashup phenomenon is all about pull-centric design. The mashup phenomenon begs a very pertinent question: can all application integration scenarios that we naturally express in push-centric language, be implemented with pull-centric designs? Is there a limit to what you can do with a mashup - something that would force you to a push-centric approach instead? If every time A needs to send something to B why not get A to publish it, with an RSS/Atom feed instead. Then B can just poll the published information as it sees fit. How far can we push (no pun) that idea?
Personally, I’m convinced that RSS/Atom feeds are a much better way to solve notification scenarios than e.g. WS-Eventing. Polling is good for you. At least on the Web, and quite probably in any similar integration scenario — which is to say, wherever you don’t control both ends of the wire.