OK, here's my guidance on the subject, barring any ruling to the contrary (I've asked for confirmation):
- If you're just linking to a forum to plug your forum, then that's a no.
- If you're linking to a site that contains other stuff, and a forum, then that's probably OK.
- If your site is ad supported (i.e. you use ads to cover costs, that's OK). Good examples of this would be free uploaders.
- If you are just out to make a profit, that's again a no.
- And finally, if your site breaks the forum rules (competitors, swearing, etc.), then that's a definite no. For example, please be careful with sites where people other than you can add content - e.g. blog sites, as these are prone to swearing etc. being posted.
I hope that helps.