Leo Bodnar has a great look at the interconnect board on the Mac mini G4:
I have noticed a missing unpopulated connector on a vertical CD/HDD interconnect board. There are also a few components that were left out during assembly. There is surprisingly big (by mini standards) empty space around that area inside the mini case.
Connector has a full Firewire bus and extra control signals on it. What could it be? Integrated iPod dock at the top of Mac mini case? Interface to some other external device? Missing internal module? Extra row of external connectors on the back?