That said, I do regard the Microsoft Natural keyboard one of the greatest keyboard designs I've used, but even that was only a slight variation on prior art. My only gripe is that I've never found one with mechanical switches (so I'm pretty sure they never existed).
PS - here is a plug for another "reinvented" bluetooth keyboard - its not foldable though :)
Edit: reviews of other mechanical ergo keyboards: http://www.anandtech.com/tag/ergonomic
I am also a big fan of the curve to the Natural keyboard, used to be my primary keyboard. However I find the combination of mechanical keys + a tenkeyless standard layout design (noticed a sore shoulder from mouse hand being far to the right) offers the most comfort for me at the moment.
Now if only Logitech would make their TrackMan Wheel still. Or maybe Microsoft would make a worthy successor? Hint, hint (I own 4 Logitech trackballs just incase they are ever discontinued completely).
(Here's how to enjoy a thumb-trackball: Push up the sensitivity to max. Give it 14 days no matter how bad you think it is. Love it forever.)