Who says you need to have cables as interconnection? In the 1980's a german HiFi company already introduced something called "Direct Contact System", where the HiFi components (tuner, tape deck, amplifier, equalizer and turntable) would be connected via a dedicated plug system that needed no extra cables (except for power in and speaker out obviously).
Each component had male connectors on the top and right side and female connectors on the bottom and left side (except for the turntable iirc), so you could arrange the components the way you liked without having a cable mess on the back.
Here are some pictures from their flyers:
http://wegavision.pytalhost.com/schneider83/schneider01.jpg
http://wegavision.pytalhost.com/schneider83/schneider03.jpg
http://wegavision.pytalhost.com/schneider83/schneider06.jpg
Sure - it was proprietary, but that's not something unusual for Apple.
So they could indeed introduce a modular computer system without cluttering the place with interconnection cables. Just the opposite - it would give them a pretty unique selling point:
Need more power? Need more storage? Need more ports? Just buy another Cube (or whatever Apple would call the boxes - it surely wouldn't be real cubes)
, providing cable-free expansion at higher speeds than any external interface could offer!
If they do it right, they could even offer one box with (additional) "standard" interfaces (Firewire, eSata, USB 3 etc.) for those who don't want to be locked completely into the Apple ecosystem.
And with the boxes designed properly, economies of scale would allow for low costs due to high volume (whether that'd translate to low customer prices is another question, though).