I think discussion forums are not a good approach for certain problems. Since they are linear, they tend to accumulate data into long threads that are mostly meaningless to most of the users and can't be reformatted to be more useful. They are good for interaction, but mostly I see this from PTC User, PTC Community, and other forums among people who would never ordinarily meet. I can see where e-mail and phone calls become more preferred among those who work closely. There's no logging-in and no extra training or discipline.
For keeping information about the design process I think WIkis are a better alternative. It is easy enough to consolidate information from e-mails and reformat discussions, while maintaining traceability of participant contributions. The best part is they not only allow crosslinks, but links outside, so that if a reader is trying to find out something they can rapidly follow both internal content and external references. It's almost like someone could build an encyclopedia with this; maybe call it Wikipedia.
The links mean that contracts, requirements, tests, inspection reports, specifications, et al can be accessed as rapidly as a person can read the information and decide what to do next. I typically harvest the text from documents to form the page content, so I don't have delays in opening Acrobat, or Word, or Excel to get the content and so I can in-line notes without affecting the original document. Harvesting also allows correcting information, particularly in Acrobat documents that have had OCR applied - some MIL specs could not be meaningfully indexed, which other systems (Sharepoint) might depend on.
The Wiki can also be accessed by anyone authorized, and they do have as detailed permissions as you want, with a web browser, in a primarily read-only format (unlike Word) but with easy purposeful editing as required. If someone needs significant training to edit a Wiki page, they probably should not have access to the computer.
To those who say that a WIki cannot work, I'd say companies have the same problem with CAD, in which a number of people have to work on a common platform, with users interacting to produce a single output. A Wiki is like CAD, but for everyone in the company.