At this moment, we've got a few different image sensor sizes.
- Camera phone sensors
- Point-and-shoot camera sensors
- Mirror-less system camera sensors
- Digital SLR sensors
- Medium format sensors
Camera phone sensors are the smallest, of course. They have to be small enough to fit around the batteries and communications hardware. They range from merely bad to pathetically bad in the world of photography. Nokia has recently introduced a camera phone sensor (in their 808) that uses adjacent pixels to determine what in the image doesn't look good and the camera and software produce a really nice 5 MP image. A few years ago, Kodak stated that they found a solution to make sensors this size much more like those of point-and-shoot cameras and a few of the camera phones actually produce good results. This Nokia innovation produces great results.
Point-and-shoot cameras use several different sized sensors and since each camera is an enclosed unit, the company can produce a lens that will work well with the new sensor. If you're only looking for a 4x6 print, these are quite good. Often though, they're mated with outlandish lens combinations like a 30x zoom. Somewhere in the middle of the zoom's range, you'll get good image quality but I wouldn't trust something this size to get an image that I wanted to save and for daily snapshots, the latest group of camera phone sensors does nearly as well, especially when 4x6 prints are what people want.
Panasonic, Olympus, Sony, and Samsung all make mirror-less system cameras with dSLR sized sensors. Pentax and Nikon initially went with tiny sensors, although Pentax recently announced their odd K01 mirror-less camera body which is little more than their K-5 in a new body without the mirror and reflex mechanism. Odd and peculiar are good adjectives for this body, as it's mirror-less for the sake of being mirror-less. It isn't small at all.
Sony has the upper hand when it comes to these sensors. They've been designing and manufacturing sensors for Nikon and Pentax for years and if they can put a APS-C sized sensor used in a $1700 dSLR into a somewhat cheaper mirror-less camera, the image quality has the potential to be good. What the camera hardware and software does with the sensor's output is another matter.
Panasonic has a lot less experience but they're enthusiastic to counter anything that Sony does. Micro Four-Thirds has 3 different sensors currently in use: a 12 MP sensor, a 16 MP, and an 16 MP multiple aspect sensor. The 12 MP sensor is also used in Olympus E-5 body, which I have. Without ranting, let me say that the 12 MP sensor is old and tired. An updated sensor of the same pixel count is on the way. Its capabilities should parallel the 16 MP sensor being used in the Panasonic DMC-GX1/G3 and Olympus E-M5. That would be a good thing, as they're once again within spitting distance of the APS-C crowd.
Of course, all of these accomplishments are reflected in the dSLR sensors for the most part. Pentax and Nikon are using Sony sensors. Canon use their own, and Olympus is using the older Panasonic 12 MP sensor, which needs an upgrade. Oops, not ranting, really. Nikon and Canon recently introduced new/upgraded bodies with 135 format-sized sensors. (Saying Full Format makes you look dim, since there are several formats.) Hobbyists won't be all that interested in buying any of these new bodies since the entry point starts at about US$3000. They will use them to beat people incessantly, due to their brand fanaticism.
Being that the upper end of the dSLR is around US$6000, medium format doesn't look nearly so bad at an entry point of $9995 with Pentax's 645D. In the 1970s/1980s, 6x4.5 cm frame cameras made medium format affordable and Pentax and Mamiya/Phase One are doing it again.
Perhaps obviously, you can put a lot of pixels into a bigger surface area and they'll be happier than trying to jam them into a smaller area. (I often relate digital sensors to a bus. When you put too many people on a bus, people become unhappy and noisy, and you end up with an unpleasant experience. Digital sensors generally work the same way. Force too much onto that surface area, and you get degradation, no matter what the salesperson tells you about more pixels.)
In any case, you end up with fantastic image quality and it's no wonder that medium format still reigns in the fashion world. Why anyone would use the Canon 1D* series or Nikon D3* series for anything but portable work is beyond me. Medium format has become more manageable so that you could use it on the street, but night shots would still require massive lighting and that's just not so portable. Kodak has been the source for medium format sensors and it's important to note that the ISO sensitivity numbers, that I've seen have moved from 400 to 800 maximum. That means you might be able to use them in dim light, but you'd be negating the image quality advantages of the format.
I'm still waiting to see a mirror-less body with a 135 format-sized sensor. Olympus recently introduced the OM-D series and its first model, the E-M5. This is a little smaller than the OM-4Ti, but the sensor is listed as 13x17.3mm vs. 24x36mm for the film frame used in the OM-4Ti. Of course, you have to make room for all the electronics and the dust reduction system, the auto focus system that the OM-series never had, and the image stabilization system. I wonder if Olympus could produce an OM-compatible camera body, though, that didn't need adapters to use the old lenses and still had the size advantage.
The problem is that Olympus never implemented auto focus in the OM-series and the IS-series, being integrated with the lens (Zoom Lens Reflex, as they called it), never won many advocates. Would they be able to squeeze everything into such a small space? If they did it, I think it would cause a revolution the way the OM-system did originally and the dSLR would shrink overnight.
So, what is the risk? You could get what's not going to be progressed tomorrow. You could get something that's already obsolete. You could get something that has no future. It may be another 15 years until this all settles, if you look at how long film took to produce easy, consistent results.