On: PubSubHubbub

That's not actually the case. It's one ping per event. You don't keep the connection open.

OK, the client must send periodic pings to the hub in order to receive updates.

Sheesh, did you bother to read the linked document?

The "client" in pubsub registers a call-back URL with the server, and the server then POSTs new data to that URL.

No periodic polling required -- that's the whole point of this.

Sorry, I was confused. I thought 'hubbub' was a drop-in replacement for RSS. Apparently, it's a meta-protocol built around RSS.