I really like this point Pete:

"In the world of marketing, I see weblogs as a customer acquisition tool, whereas forums are a customer retention tool."

Very true. I had also planned to add that I think weblogs talk "to" customers and discussion forums talk "with" customers.

Also I think it's important to note that it isn't just a matter of adding a discussion forum (or weblog for that matter). Like any tool, it is how you use it that matters.

Online communities don't develop because of tools, they develop when people use the tools to effectively serve an existing need.