From the photos I have seen, I had not been able to tell much about how the C300 sensor is protected from dust and dirt when lenses are changed. I had a chance to see one up close at Showcase Video in Atlanta, Georgia yesterday and shot this quick look inside...you can see the Infrared glass filter protecting not only the sensor, but also keeping dust off the internal ND filters.
The video also shows the smooth action of the glass ND filters as they silently glide in front of the sensor. When you operate the camera, there are two buttons, "ND + and ND -" that allow you to go from 2 stops to 4 stops to 6 stops and back around to nothing. You can keep pressing the "+" button and the filters will cycle through from 0 to 6 stops and back indefinitely. Also, there are three separate filters, so the camera does not "stack" filters internally. The IR filter in front of the sensor and ND filters ensures that no matter how cheap a lens you put on the camera, you're still protected against color shifts.
I love the fact that they included all that goodness behind the lens, so you can get the most resolution out of your lenses by shooting without any filters in front of the lens and yet still have great exposure control. This is the real genius of this camera to me, and it is why you could shoot without a matte box if you have a clip-on French Flag to control flares.
Also, the camera was on during the entire time I was there for the demo and even in a quiet, carpeted basement room with acoustic tile ceiling, I could not hear the cooling fan. The air intake ports on the camera are almost impossible to find; they look like the perforated grills of external speakers on cheaper cameras and are tucked in out-of-the way places on the body; I could not even find the upper port where the air leaves the cooling system of the camera. It is truly a silent running camera.
Everything on the camera just feels very solid and well made. All the buttons feel damped and weather sealed. Taking the camera off the tripod, it feels great to hold by the base and with the side grip, but trust me this is a two-handed camera only. With the top handle off but the monitor and sound unit attached on top, it was very solid, easy to hold at chest height with both hands, but not as comfortable trying to hold with just one hand and the other on the lens, as I might operate my 60D. It's easy to hold, but hefty. I imagine the most comfortable way to operate this camera off a tripod would be with one of those Manfrotto monopods with the fluid head on top and the 3 feet at the bottom, like a lot of DSLR wedding photogs use.
Hopefully this video is interesting to those of you who have not seen the camera up close yet and were wondering like I was about how the sensor remains protected when you swap lenses.