I think it is absolutely true that a corporation is better of with comments-open weblogs running. Conversation is always good, and the naysayers who say that you never know what negative stuff will get said or what loons will abuse the comment system clearly haven't been paying attention to the worldwide mass experiment in voluntary collaboration that is building e.g. Wikipedia.
But I still have doubts about the consequences for the corporate bloggers of actually blogging for their corporation. To keep it short, I believe that people's distrust of corporation will rub off in a distrust of the corporate blogger. I know it's my personal instinct to think so. People accept roles in corporations and do things they wouldn't do if they weren't there. That's all fine and dandy when I am able to recognize that they're in that role. I think corporate blogging blurs that.
I think I'm also repeating myself. You might want to follow the discussion attached to this post.