Some of the most beautiful cases I've ever seen for the Kindle have just been created out of oiled leather by Etsy member, hmcurrie:
http://www.etsy.com/shop/hmcurriers
We might not be able to petition big companies to create cases for the 350 and 650, but it might be possible to convince smaller crews and individuals like hmcurrie if they receive enough requests. Now that the official Sony case has become a rare bird, I can't think of a better option for the same price Stateside than if the gorgeous leather and clasps being offered by hmcurrie were available for Sony readers.
Edit: Now they are available:
Tiny version of the original image of the HMC Kindle case that started this discussion: