I really hope so, but both of my friends who attended the open house were also accepted by UCB (which means Duke must want them as well). I know that one of them got a letter via email before, not sure about another. As mentioned by a post above, I also think the invitation is just a sign of acceptance. However, Duke does have a second-round open house! Maybe there will be more offers coming soon! I plan to ask tomorrow. I would keep you updated if I get any info tomorrow.