Dan's email to me said more or less "Do whatever you fancy" and made it clear that he wasn't looking to make any money out of it.
I think that there are a couple of different approaches.
1) The forum is a community where people who share a trade can swap advice, stories and have a laugh
2) The forum is a valuable business tool, with brand equity and value in its own right and really quite cosmic google rankings for certain search terms
I can see a bit of both. Obviously when I am in sponsor mode, I am thinking more of no 2). The honest truth though is that the business angle wouldn't be enough to keep me here. If it was just about business, I could pay Dan for banner ads, and buy all your personal email addresses from CIPHE or GasSafe.
I think the forum has more value as a community than as a business, and that many of the business benefits that it has are actually a side-effect of the community element, not the other way around.
But thats just my view - one amongst many.