Thanks Maurizio,
I'm wondering the same thing that John asked -- what would make me decide to do it one way or another. I was just having a conversation with a coworker, and we were thinking maybe for our purpose we'd go www.forum.yourdomain.com -- for this reason:
The forum will be about 1 type of racecar, whereas the company is a racetrack that sponsors many types of racing, as well as being a sales point for this one racecar. In this case, I believe the company owner is trying to engage people for that specific vehicle.
The unanswered piece of this for me is whether there is an SEO implication one way or another. From what I understand, forums usually use nofollow on links, which means the forums.yourdomain.com doesn't really help the core company site -- other than potentially sending people there via topical interest and mentions by members (so it's a marketing play). To your point, if the forum is www.yourdomain.com/forum, then the content actually belongs to your site -- so ....... hmmmm. I'm still torn. Hopefully I'll get more input.
Thanks for feedback. It's helpful to hear what others are doing.
Cheers!
Kristy