"OK, You asked for it – a digital camera with interchangeable sensor. Why buy a whole new camera every time a new generation of sensor comes along – you asked, and many are still asking. The answer is – because few companies have the moxie to make such a device. Ricoh does, and has.
But, until now Ricoh didn't quite get it right. The reason is that the sensor lies between the body and the lens, and not having a line of interchangeable lenses (at the time), Ricoh decided to sell their sensor models with lenses built in. Humm. Not so attractive; A – because it means that one is stuck with just one lens, and B – when you upgrade the sensor you lose the lens as well.
With the just shipping (September, 2011) Mount A12 (catchy name isn't it?) Ricoh has taken their GXR interchangeable sensor concept and made it click (pun intended). Instead of a sensor with a lens, to attach to your GXR body, they have produced a sensor with a lens mount; a Leica M lens mount to be precise..."