The Mac Media Center forums had this to say about the power connector of the Mac Mini potentially having more "lines" than needed:
It has been speculated that this port could be used to stack something ontop of - or under - the mini. For instance, have you heard of the rumored "asteroid" audio input/output device? its supposed to be about the shape of the mini and fit snugly ontop of it. If you could daisy-chain devices to the mini's "power" port like firewire, it could be extremely useful (and stylish too).
This rumour stuff is fun!