I'm referring to the "free" slot as second slot, the first slot is the one which is already occupied by the built in wifi card.
so the second slot seems to be locked to any card except some WWAN/3g cards (don't know about pcie-ssd or other cards like the mentioned crystal-HD)http://www.netbooks.ph/forums/viewtopic.php?f=56&t=1756
Any wifi card you will put in there "seems" to pass the bios check, but it just won't be detected by windows, so I guess it's just deactived/locked by the slot.
also make sure you buy a genuine 5100 made by/for lenovo
(or at least the ebay auction should say so or that it's working in the x300 thinkpad).
dell oem versions or any other manufacturer's version of the same card won't work.
the first 5100 I picked was the cheapest offer on ebay (like 12$ - dell oem), the second one said it's a genuine lenovo and was like 24$...
it's kinda strange that it doesn't have any fru number on it, but it works.
I have encountered some kind of bug, I don't know if it's being caused by some driver conflict or by windows 7 which I'm using,
but when I transfer data over wifi now my system performance drops drastically... it's like 100% cpu load...
Surfing and transferring smaller files works just normally.