As we already have it on the iPad, iPhone and on every MacBook, bringing multitouch to the desktop seems to be just the next small but inevitable step.
I don't think this trackpad will be anything really fancy. Won't work with a stylus, won't have an integrated display etc. etc... Rather a bluetooth (maybe slightly larger) incarnation of Apples notebook touchpads.*
Look at Apple's keyboard offerings:
There's a clear trend towards
unification in Apple's input devices:
Mac keyboards have been essentially the same for quite some time now:
Same switch technology, same feel, same sizes and measurements, same keyboard layout. Note how they moved to keyboards without integrated number pads as the default option for their consumer desktops? The last two incarnations of the iMac shipped with what you could call "notebook keyboards". Not without good reason, if you consider that notebooks have replaced desktops as primary computers for many people. So why should the touchpad be anything greatly different than on the MacBooks?
* Which, of course, doesn't prevent Apple from touting this trackpad thing as the biggest and magiciest innovation since the mouse
I'd guess it'll be introduced with the upcoming iMac revision.