The issue with lenses is a good point. On the other hand I would guess that the flange distance of the present Alpha-mount would preclude the use of existing cine lenses anyway. Sony is said to work on a short flange alpha mount (like micro four thirds), so fitting cine lenses may be possible.
Yes, using that shorter lens mount of the forthcoming "Sony EVIL camera" would make far more sense. That would then lead to using a sensor in roughly APS-C format (close to cine-35mm format) for which that mount is designed, like the forthcoming EXMOR HD APS-C sensor or Sony's professional video Super 35mm CCD. Using a 36x24mm sensor would still make little sense, as most cine-camera lenses are designed for smaller image circle needs, so vignetting problems would be rife.
On the other hand, it seems that having full frame is what makes the Canon 5D popular as a movie camera.
Popular for adding movie options to a still camera, and for being far less expensive than any dedicated video camera with a sensor larger than about 2/3" format, but no DSLR comes close to matching the ergonomics and usability of a camera designed for motion photography, and with a sensor adapted to video meeds through appropriate AA filters and such. So far, putting a sensor of any given size into a professional motion picture camera body pushes the price up vastly higher than a still camera of similar sensor size. Canon's new high end 1/3" format camcorders cost over $6,000, Sony's 2/3" SRW-9000 costs $105,000 and its new Super 35mm PL-mount model SRW-9000PL will be about $125,000.
Clearly, high quality camcorder pricing is affected by a lot more than sensor fab. costs!
P. S. I just checked and PL mount has a fairly large flange to focal plane distance of 52mm (to allow big spinning mirrors?) So in that respect, PL lenses could be adapted for use on a 35mm still camera lens mount. The APS-C "HDSLR" cameras are good candidates, matching the image circle well too.