What st0w said is right. Apparently the apps get downloaded in batches. When I stopped by Dean Franco's office last time, she had a stack that she said she had just gotten. Obviously, not all of those apps could have been submitted at exactly the same time! Also, she said that there were other screeners looking at some of the apps, not just her. Some screeners are going to be faster than others. So it's the luck of the draw which batch your file gets downloaded with and how long your screener takes to get through it.
Someone else who was worried about not getting an invite yet PMed me earlier. I told them that even though it seems like everyone on this whole thread has gotten an invite except you, there are still a lot going out. Even though it sucks to wait, sometimes you just have to. Believe me, I understand your pain. When I applied, I was complete around maybe the end of August. But I got invites to both programs around Halloween, interviewed at both in January, and was accepted to both at the beginning of February, in plenty of time to attend the second looks. So if you're not the very first person on SDN who gets an invite, it doesn't mean you won't get one. As long as you haven't heard anything, it means they haven't made a decision yet (and maybe didn't review you yet). Try to look at the bright side. I guarantee most of you will not have to wait as long to hear about an invite as I did!