This means that identi.ca, the server operated by status.net, has the feature, as well as all other sites they operate. I assume it will be baked into a subsequent open source release (status.net is GPL software).
What does this mean? Well, when I post an update to my account on identi.ca, any cloud-aware aggregator will receive an update notification. River2, the aggregator I've built for Frontier (it runs in the OPML Editor) has support for rssCloud.
For a demo here's a screen shot of an update I posted to identi.ca. Note the time of the update. I immediately refreshed the home page of my River2 server, and there's the update. Elapsed time ==> 12 seconds. That's what real time means. ">
It's also a holy grail for the idea of a distributed loosely-coupled network of Twitter-like services, linked together in real time using RSS. (What a mouthful!) It's very elegant and lightweight and it works today.