I am currently looking at adding a mini PCIe header to the Shield in order to add a laptop form-factor WiFi card.
The pads for the header are unpopulated. From what I understood, it was going to be added in the later revisions of the Shield (Called the Shield Pro?) but, well, we know that story.
That being said, the PCB seems to be enabled to handle it already and the header is just missing. The kernel init's the PCIe bus and sees that the slot is empty (aka Missing), so I'm hopeful it's just the fact the header isn't there.
There is an unpopulated U2 chip-slot on the reverse side, but I don't know if that is for power, or something unrelated. The mini headers are $5 or less new and I want to see if I can find a local shop who can surface mount one to the pads.
This is simply a "Hey, check this out, it might be interesting!" post at the moment, but I've got the spare Shield to brick if I have to, so who knows..