How badly do people want interchangeable lenses on compact cameras? We're going to find out soon enough: Panasonic and Olympus have announced a variation on their Four Thirds camera system designed specifically for non-SLR cameras.
On one hand, this will facilitate interchangeable lens cameras more compact than would be possible in a dSLR. By jettisoning the mirror box and through-the-lens optical viewfinder, the two companies hope to make thinner and lighter cameras--thinner than the Olympus E-420, the smallest dSLR on the market. The new lens specification allows the lens to sit about 50 percent closer to the sensor and has a smaller lens mount.
On the other hand, the sensor size remains unchanged from the regular Four Thirds, 17.3mm x 13.0mm, which is pretty large for a non-SLR, and therefore provides what might be too high a lower bound on the width and height of the bodies.
There's no inherent technological advantages to the Four Thirds system, which is one reason it never really took off as an alternative to all the proprietary mount and aspect offerings from companies like Canon, Nikon, Pentax and Sony/Konica Minolta.
Though there remains no advantage to having a Four Thirds aspect sensor in a compact camera, this time Olympus and Panasonic are proposing a lens-mount standard where there's no existing alternative, and enabling new camera features to boot. That gives Micro Four Thirds a much better chance of gaining traction. The system's lens mount is slightly smaller than the dSLR system--by about 6mm--and it allows for adapters from the larger mount. However, most Four Thirds lenses are pretty heavy, and would probably feel pretty unwieldy on the compact bodies.
The concept sounds great in theory, but a lot depends on both execution and pricing; we've got no idea how much these cameras will run. And while it's true that many people like interchangeable lenses in theory, when faced with the reality of carrying a lot of pieces they balk. Also, a lot of upgraders to dSLRs do it for the speed and photo quality, not the lens flexibility.
It seems the most likely segment to adopt these products would be enthusiasts currently buying cameras like the Canon PowerShot G9 and Panasonic Lumix DMC-LX2, but the photo quality has to hit a certain level for these users. And, ironically, Panasonic is it's own biggest competitor for something like this--do you really need the interchangeable lenses when you've got a wide-angle, 10x zoom lens on a compact body already?