The client side stuff might be ready, but they may still be working on some of the iCloud stuff. I've certainly started seeing some unexpected error codes in iTunes this morning so I suspect they are working to put everything together for the launch tomorrow. You'll be surprised how much might be involved in delivering this update to everyone.
How many people would be happy if they updated their devices today and a bunch of errors started popping up on their devices because iCloud stuff wasn't ready or updated properly?