A round sensor will, as some of you have indicated already, be expensive. But let's play with it a bit anyway. Expensive doesn't mean impossible remember!
If you had this round sensor, - you could easily blind the areas you didn't want. So you could, with a snap, take panorama, normal or sqare pictures. All of the image could be recorded in the RAW (the blind apply's for the viewfinder as well as tell the RAW what not to show). So you would afterwards be able to get out more data if you want (you redecided, - wanted a normal instead of a panorama anyway, - just like now you shot at a wrong white balance).
Of course, - if you didn't save the extra pixels you could also make it digital zoom with this, - so the less pixels you store the longer your lense seems, - and the faster can you store the image to the card.
Also there are no reason to turn the camera to take portrait pictures, - so the flash will always be on top and not at the side. A nice (although smaller) benefit too!
Really I could see lots of nice possibilities with this, - but I don't really think it will be. Unfortunately.


