Previously I was thinking that mobility might be a fly in the ointment of PubSubHubbub and other Pushbutton Web approaches. There is applicable prior art though. Basically you need an intermediate, reliable server that the mobile client talks with. Then there are two approaches that could be used.
First, good old SMTP e-mail’s store and forward model could work well.
Second, the sync models of webfeed aggregators like NetNewsWire on the iPhone might be a better fit.
Of course, now the issue is how to push through the intermediate server when the client is connected, so you maintain the “real-time” illusion.