James at Column Two has posted some good advice on the use of discussion groups inside a corporate Intranet.
A couple of points with which I agree:
To summarise: discussion groups only succeed if they either support an existing community, or meet a specific need. "Build it and they will come" simply doesn't work.
The good news is that the technology isn't particularly important. There are a huge number of excellent (and free) discussion board packages about, the trick is just to find one that is easy to use.
One point I would add in terms of implementation is the use of a notification or alerts system. Discussions groups change quickly and people can feel left behind and give up on keeping up with the discussion. By using email or RSS notifications, the company help ensure that people have a resource for keeping up with the flow of the discussions.