I don't like using the proximity sensor. I know that because I hit it all the time with that other app that allows for video that allows you to control focus and etc. I find that it doesn't work that well and is not clear. A button I think makes more sense.

As for dark frame subtraction I don't know anyone that does that for each individual photo. It is simply impractical at the user level. I actually submitted an RFE for dark frame subtraction and fcamera can do that by forcing the shutter closed and then taking a photo for the same duration. This could be for longer exposures (longer than 1/30 or 1/2 second whatever the right threshold is for the N900). It is much easier if the camera handles that by itself and it can do it without having close the lens protector. It is a good idea though but would be handled differently. Plus you have to find a tool that does dark frame subtraction and you have to differentiate what is signal noise and what is just random noise. Dark frame isn't supposed to reduce random noise, but only the noise like signal noise that is predictable and recurring.