Late-2007 Mac Mini (A1176) Unsoldered Mystery Connector on Interconnect Board
There were rumors early in the mini's life that there was (at some point) supposed to be a built-in iPod Dock on the top. They were even supposed to have gotten far enough in the design that they left the headers for the USB and FireWire connections on the board. Those may be them. (Here's a The Register article on the subject.) 

To my knowledge, nobody's proven or disproven this rumor. If you, in your explorations, find an answer, do post it here!