The first rssClouds were implemented in 2001 in Manila and Radio UserLand, two products from my company, UserLand Software. All three sides of the protocol were implemented.
In 2002, we spun out the server side into an open source release called Radio Community Server, which had other functions that supported communities of Radio users. We operated two such communities, one for our own users, and one in partnership with Salon. A bunch of leading weblogs came out of these services.
The aggregator in Radio UserLand would receive instant updates of feeds of other members of these communities, but not many people noticed. We also had the equivalent of Twitter's "retweet" (that's why the <source> element was added, so the feed would link back to the originator, a feature that Twitter doesn't have).
In any case, polling became the norm, and the cloud functionality didn't get much use outside of our communities.