Thanks for the quick reply, I think I missed something in the wording of my question.  I understood (for the most part - the extra detail was much appreciated) why the heat was dissipated from the back of the board.  I was looking more to understand why the traces for the USB power were on that side of the board and not on the side with the components.  After actually checking the traces again, I realized that you did design your board opposite what ASICMiner did and the heat sink will actually face up when plugged in (at least on the two machines I have in front of me).