Chris McGrath of One Intranets has a nice set of 6 Steps to Effective Discussion Forums on your Intranet, which I find fairly interesting. He's got more details in the full entry, so I'm not giving away the store here.
- Attach a forum to every page.
- Provide an aggregated, prioritized view of discussions.
- Integrate forums with the company directory.
- Strip the interface to the basics.
- Signal participants when a post is made.
- Engage your organization's thought leaders.
As I read through the article, it's clear these ideas are influenced by the current thinking around social software. And they apply outside the intranet too.
The other thought I had was whether in-house forums work at all, unless you are a very tech-centric company. Many companies have the capability, but they certainly don't integrate forums the way McGrath suggests. Of course, maybe this is why most of that capability goes underutilized.