Well, I can give a pretty good guess that apple will be showing off the new id gaming engine. ( check out my previous post.)While a redesign of the iMac for tomorrow is very likely, and may not be the only announcement, I would strongly advise that all of you who are posting lists of everything that you think will be announced not get your hopes up. Don't forget, last time Apple hosted an event of this sort, all we got was an unimpressive update to the Mac mini, the introduction of the iPod HiFi, and the leather case for the iPod that I think sold about 14 units.
Hey, check out this youtube vid of John Carmack talking about his new gaming engine for the mac. At around 1:55 he brings up how he would be previewing the engine in august and as well as a mac related announcement that he couldn't go into. So is there any possibility of this being that announcement in august?
http://www.youtube.com/watch?v=dhArACnKYys
The only "Top Secret" features I can think of would be to team up with Nintendo and add a scaled down wiimote as a the new apple remote.(Perhaps the Top Secret features? Do they even exist?!)