Yesterday, WordPress.com activated PubSubHubbub on the more than 10.5 million hosted blogs.
PubSubHubbub, or PuSH, is a service that pushes updates to blog readers rather than relying on a traditional pull. In other words, when a blogger publishes a post, rather than waiting for Google Reader to check for updates, PuSH tells Google Reader that the blog has been updated.
According to Joseph Scott's announcement, the updates are sent out within a few seconds - so you better be absolutely sure you're ready when you click publish.
For bloggers running their own WordPress.org software, Scott said a new PuSHPress plugin will be out shortly. This one one will have it's own built-in hub.
You can learn more about PubSubHubbub at the project Web site.