I have been wanting something like this for ages (wide angle converter that's behind the lens)

Meuh wrote:

Yup, I don't think you would get the DoF of a f0.9 lens, but very similar angle of view and dof of the same lens used with a FF sensor but increased light gathering since the image circle is compressed.
Nikon already had a digital SLR that had this sort of thing built in.
http://en.wikipedia.org/wiki/Nikon_E_series