All blogs on the WordPress.com platform and any WordPress.org blogs that opt-in will now make instant updates available to any RSS readers subscribed to a new feature called RSSCloud. There is curretnly only one RSS aggregator that supports RSSCloud, Dave Winer's brand new reader River2. That will probably change very soon.
RSSCloud is an element that's always been present in the RSS 2.0 spec but has drawn new attention with the rise of interest in the Real Time Web. The element was just added to the WordPress code this afternoon. The implications of this big vote of support go beyond reading WordPress blogs; this is the kind of traction that new technologies can leverage to gain support in many different applications.
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Supporting feed readers will now be able to request updates from WordPress blog feeds as soon as they become available, instead of polling a server periodically to check for updates. The feature is already being rolled out, several WordPress users report seeing the cloud element in the source code of their RSS feeds.
The difference is, instead of the typical feed reader that checks for updates every few hours, this one checks for updates in real time. In other words, every few seconds.