Personally, I prefer direct links in my footers of the forums, as seen here (please keep in mind it's a work in progress):

http://www.lifesupporters.com/modules.php?name=Forums

Now that my forums are climbing the PR charts, I'll likely add them to a comprehensive ad purchasing program that I'm designing for my sites. I've had enough of adsense wasting space and possible revenue as well so I'll likely sell the space in the header of the forums as well.