Update – 5 Nov 2009:
These features are now available on WordPress.com as well – http://en.blog.wordpress.com/2009/11/05/rsscloud-update/

Version 0.4.1 of the rssCloud WordPress plugin is now available. The biggest change is adding support for the domain parameter in notification requests. This means that rssCloud updates processed by the plugin are no longer limited to being [...]

RSSCloud support has been enabled on all WordPress.com blogs. If you are running a WordPress.org powered blog you can do the same thing with the RSSCloud plugin.
So what does this really mean?
From the point of view of WordPress and this plugin there are three main additions:

Adds the <cloud> element to your RSS2 feed (more [...]