PubSubHubbub, for those not in the know, is a server-to-server protocol for notifying interested parties of events they're interested in. Google now made it possible to subscribe to three types of YouTube user events via PubSubHubbub: "video uploads, new subscriptions, and video favorites." The subscription requests need to be made on a per-user basis, so you'll only receive updates for specific users that you're interested in.
Your code can look for PubSubHubbub hub address in href attribute of the top-level
<link rel="hub"> element in uploads, subscriptions, and favorites feeds. For e.g., the feed http://gdata.youtube.com/feeds/api/users/GoogleDevelopers/uploads?v=2 exposes the hub address in
<link rel="hub" href="http://pubsubhubbub.appspot.com"/> element.