This shows a user on Cliqset mentioning a couple of buddies using other services (StatusNet and a scrappy startup called Atollia), having information flow between the sites, and having mentions be pushed the other services. The big take-away here is that these services don't need to pre-register or federate with each other any more than they need to federate to send email back and forth; it all Just Works. [abscractioneer.org]
It all just works with Webfinger auto discovery and Salmon. The video was part of the social web session at Google I/O. sweatinvestor.com has some additional notes.