Upon further investigation I don't think it's the authors of UBB or IE.
I think the root of the problem is that the pprune cookie comes from pprune.org but the site url is pprune.com, so IE6 sees it as a third party cookie which is rejected by the medium strength privacy settings.
The problem doesn't arise for those using
www.pprune.org as the site URL.
From the IE6 help file on the subject:
A first-party cookie either originates on or is sent to the Web site you are currently viewing. These cookies are commonly used to store information, such as your preferences when visiting that site.
A third-party cookie either originates on or is sent to a Web site different from the one you are currently viewing. Third-party Web sites usually provide some content on the Web site you are viewing. For example, many sites use advertising from third-party Web sites and those third-party Web sites may use cookies. A common use for this type of cookie is to track your Web page use for advertising or other marketing purposes. Third-party cookies can either be persistent or temporary.
[ 10 September 2001: Message edited by: Seat 32F ]