Lets say you are starting a site in a highly competitive market. Lets also say you were creating it for a product that hasn't even come out yet but the company promoting it is all over TV, the web, etc. Would you start with a forum and build outward to a distribution site, or would you start with a site with news and work downwards towards a forum? The goal is, of course, to bring in targeted users and gain as much market share as possible. I'm just not sure which is the best approach.
Get a mailing list of people who are interested in the product. Once the list is nice and big, email them all saying, "hey guys! I hoped you liked my site. I am now opening a forum..." and watch the people flock in.