Discussion Forums on the Corporate Intranet?

WallyHartshorn asks: "We are about to implement discussion forum software on our intranet at work. In preparation for this, I have been given the task of writing up a policy document. I'm curious as to what kinds of experiences other people have had with discussion forums on their company's intranet. (Just to clarify, I'm not seeking software recommendations) How much do they get used? What types of discussion areas seem to be the most useful. Forums for specific committees? Forums for specific departments? When a discussion area is to be used for a specific committee, is it useful to make the discussion area open for anyone to participate, or does that disrupt the discussion among committee members and make them less likely to voice their real opinions?"

"We are also going to have a general discussion area for socializing and discussing topics unrelated to work (weather, sports, movies, whatever). Is this generally a good idea, or does it cause too many problems and result in too much wasted time?"

Companies that have an intranet may benefit from having such a system, however like any tool, these can be abused (loss of productivity and political infighting for example), and such issues should be taken into account before the system is deployed. What policies can be attempted on such systems which will mitigate some of the drawbacks of discussion forums without curbing the benefits?