A forum will generate traffic if it's good, useful, and relevant... and set up in the right way that it's search engine friendly. The trouble I've found on forums in the past is that it only takes one bitchy customer to turn it in to a slur campaign - we had one a couple of years ago when the company I worked for had an online support forum - before you know it, one user who had a gripe because we wouldn't support him until he apologised to a member of staff (he told the support call handler that she's a stupid c*nt and a woman couldn't know about IT) - after he refused to apologise, he then went on our forum slagging off everything about the company, and all of a sudden every customer complaint from "My hard drive only shows 7.5Gb and it should be 8Gb" to "The invoice arrived a day early this month, I think they're desperately trying to claw money in"... We weren't as prominent on the net as companies are today, and Google was nowhere near as aggressive as it is today, but nowadays i imagine a company could come to serious
PR harm.