My friend Jennifer pointed out an interesting blog entry over at Curt Monash's site about the use of Twitter in the enterprise. He cited four interesting use cases: real-time short communication, real-time wide outreach (i.e. broadcast), IM integration, and outside-the-firewall communication.
I think he nailed those four - enterprises could use twitter effectively to reach out to employees. But I think there's an overwhelmingly more powerful use case for twitter in the large corporate world - it's that of communicating humanity to those in the enterprise. This is especially true of those in senior management.
One of the most painful and difficult things about the corporate world is the facade that most senior managers put up - there's so little humanity that trickles out of the CEO's office in most companies. Technologies like twitter would enable (and I might dare go as far as to say force) management to be more human to their teams. And that is almost always a good thing.