Congrats to Adam Gutterman for finding the cake. The cake is hidden on an internal ground layer, printed in “negative” (i.e., lack of copper on a copper plane). The visible-light photos I make of the motherboards are all taken with front-lit scanners, so the negative image is virtually invisible when viewed in that way. There’s also a solid copper ground plane behind the cake as well, so you can’t simply hold the board up to a light to find the cake (hence the larger bounty). The only two methods to find the cake are to either X-ray the board, or to mechanically delaminate the board using either knife or sandpaper (or both) to reveal the cake. And for the record, the region where the cake is located is electrically inert, such that defacing the chumby logo to reveal the cake does not damage the function of the device.
That was fast!