
Apple will get rid of the chin below the iMac to match the size and (slightly thinner) look of the CinemaDisplay, which will also get an update to support ThunderBolt. The size will stay at 27".
The CinemaDisplay will get an update to give it an extra Thunderbolt port to daisychain more devices (which will also have two Thunderbolt ports for daisy-chaining) and you'll be able to use two extra cinema displays along with your iMac for a total of 3 displays. The CinemaDisplay will further get rid of the USB in their squid cable because this is supported through the Thunderbolt connection (at least when all the MacBooks have been updated to Thunderbolt).
Apple will work with intel to secure the first batch of their new 6 core Sandy Bridge i7 CPU's. Reserving the single and dual octocore i7 processors for the Mac Pro but still making the top iMac model compete with all but the fastest current gen Mac Pro's. Obviously the graphic card will also get an update to facilitate the optional extra display.
At least the high end iMac's will get a flash storage in addition to the normal 3.5" harddrive to hold the OS and programs/apps, the price should now have gone down enough to make it cheaper to just solder some flash storage onto the motherboard than an awkward 2.5" SSD installation in a cramped computer.
Most other improvements will be from Lion.