Yammer is Twitter but inside your company. Before I started using it I dismissed Twitter as a huge time sink and not worthwhile. After using it a bit I've found:
#2 is important because it fertilizes accidental serendipity, which can be a hugely powerful force. Blogging goes a way toward this but only in a heavier fashion. And there's typically too much baggage when posting a blog entry to make little thoughts worthwhile.
Accidental serendipity is a concept I've been interested in a while. It's what makes working in an office with other people worthwhile -- you pick up in your peripheral hearing a phrase and follow-up to learn more, or step in to help out.
It's very difficult to do well without near-human contact, but for something technological twitter does a good job. You need to do some occasional gardening of the people you're following, and it's useful every once in a while to follow messages tagged to another user you're not aware of so you can expand your population.
Anyway, it seems like it would be awfully useful across an organization -- you can't share an office with everyone. The persistence and searchability could be useful too, but IMO they're gravy. (I heard one of the Java Posse guys is using it and should probably post a message on the group...)