Probably that the biggest secret feature is Multi-touch screen input, just like the iPhone. So they will present new monitors that can do that at the same time they are going to release Leopard.
btw: There was a patent accorded to Apple related to this a few months ago, and it was attributed to one of Logic main programmer, so again this must be a reason why Logic 8 was still not shown yet.
What If There Are No Top Secret Features in Leopard?