Brett Slatkin on the difference between PubSubHubBub and rssCloud:

from http://jy.typepad.com/jy/2009/07/pubsubhubbub-rsscloud.html:

In a nut-shell, rssCloud's subscriptions are merely a way of redistributing pings to subscribers. We think this is still on the publishing side of the problem and does not simplify the life of a subscriber. With rssCloud, subscribers must re-fetch the feed to see if it's changed. In contrast, Hubbub delivers the actual changes to the subscriber so they have no more work to do. This makes it much easier to subscribe, and has some nice properties when it comes to scalability.