There are lots of predictions about what Apple will announce monday, many of them sparked by mysterious boxes being delivered to dealers around the world.
I suspect there will be some kind of docking station with keyboard & display for the new iPhones. Since Apple is now referring to the operating system as ‘OS X Leopard’ instead of ‘Mac OS X Leopard’, there will most likely be some non-Mac device that will run it (actually it already runs on several non-Mac devices: the Apple TV, iPhone, and iPod Touch).
The iPhone already has a lot of the Mac’s capabilities, so it isn’t much of a stretch for them to have a docking device with a keyboard & display to make it a full-fledged computer. Another alternative would be something similar to target mode or portable home directories, where a Mac can use the iPhone as their home directory when it’s docked, so you can easily take your setup anywhere and use it on any Mac.