Okay. Here’s the issue that is causing this:
1. WordPress has a security feature to make sure that comments are posted from the current site. It does this by checking the HTTP Referer header. Typically, browsers set this header to indicate the page that a user is coming from (i.e. they click a link from www.a.com/foo to www.b.com/bar, so the www.b.com server sees a request for the bar page, and it sees a referer of www.a.com/foo)
2. Some privacy tools prevent the HTTP referer header from being sent (some firefox extensions, Opera, and some personal firewalls/antivirus products have this option, or leave it enabled all the time).
Thus there are two solutions.
1. The blog host can disable the referer check in WordPress by opening up the PHP code and commenting out the if statement that has been mentioned above in this thread.
2. The person trying to post can disable whatever is stopping the referer header from being sent (whether it is a firewall, antivirus, or a browser feature/extension).
In reply to tcpayne, perhaps IE is using NAV as an HTTP proxy?
