I recently covered Socialcast, a "Friendfeed for business," and liked it a lot. It takes emerging social interaction models that people are just now getting accustomed to and adapts them for business.
Here at TechCrunch50, the idea is also in evidence with Yammer, more of a "Twitter for business" than Socialcast, since it doesn't seem to be able to pull in external feeds the same way.
However, users can have threaded discussions, as they can on FriendFeed. Users can also use "hashtags" for tagging topics, and they can follow just those tags. Useful if you want to follow a project, but not necessarily all the people working on it.
Yammer will launch with a desktop AIR app, as well as iPhone and BlackBerry apps, and an SMS interface.
The base product is free. Enterprise versions with admin tools and security features will cost you.
I really like this concept, but my fear is that this kind of product is too easy to build (especially on a workgroup scale, as compared to the consumer scale Twitter has struggled with). What I don't see is a blocking business strategy. But I still like it.
The service is now live.