Olympus Pen F lenses should be added to the list of those which work on Nex but not on M. And they are spectacular little lenses. I just recently tried the 25mm f2.8 and in most of the frame it was as good as the Zeiss ZM. The edges were a little less sharp wide open. I have the 38 f1.8 on the Nex and it's also a cracker of a lens, extremely sharp.
I'm thinking about moving back to the Ricoh for a number of reasons, but I would miss the Pen 38mm. My other favorite on the Nex, my Contax G 45/2, is off to be converted. Digitalintrigue is sending a bunch of lenses over to Taiwan to get converted to M, you can read about that in the Contax forum. It's not cheap but the final price with the lens donor doesn't exceed the Zeiss ZM 50/2 Planar, which i found to be just okay. The 45/2 Planar, on the other hand, is a wicked, wicked lens.