Today, Olympus and Panasonic announced a new mirrorless format based on Four Thirds. Sensors in the new system are the same size as they were in the original Four Thirds specification (18 x 13.5 mm), but Micro Four Thirds cameras will be considerably smaller due to the removal of the mirror. In addition, lenses for the new lens mount will also be smaller. The drawback is that without a mirror, optical viewfinders are not possible, and framing will have to done via Live View or electronic viewfinder. Micro Four Thirds is compatible with Four Thirds lenses through the use of an adapter.
This is particularly exciting for underwater shooters. A large sensor in a small package will allow us to maintain low noise in images while minimizing the size of a housed camera!