I don't usually get into wild speculation on this site, but what the heck, let's give it a shot!
So this guy's a little weird, huh?
There are 2 things to note about this. The first, which everyone seems to have noticed, is that it's kind of out of place. While the rest of the circles are all working together to create an Apple logo, that one is just hanging out there on its own.
But then there's the thing I have only seen brought up by @markdmill on Twitter, maybe it's a home button. At first I was skeptical, but I changed my mind after getting up close and personal with the graphic as I made an iPhone wallpaper for the event. See, there are 43 rings on this image1, and 42 of them are circles with a 5 pixel border2. There's just one circle that has a different border, and it's this one.
So the border is 3x as thick as the other ones. The interesting part of this is that if you were drawing a circle to represent a home button on the iPhone, you would make it with a 15 pixel border as well.
What the hell does it mean? I really don't know, but I wouldn't be surprised if we walk out of next week's event saying "of course it was a home button!"