Even with digital, format size does matter, and not just pixel count. Camera shake, as well as all manner of lens abberation and focusing error are ALL magnified from a smaller sensor, even before one begins to see pixel effect.
Personally, I seem to have a hell of a time holding my DSLR steady. I have much better luck with either my Mamiya 6, Bronica S2, or Leica.
You've got 3 explanations above why camera shake is not
magnified with the smaller sensor provided the focal length of the lens is adjusted to provide the same field of view
. However, it is
true that increases in pixel count put greater demands on the lenses used. That applies across all formats, including the P65 which has a pixel size similar to that of the D3X.
Years ago, questions used to arise about how the 1/FL rule applies to the cropped format camera. As a guide for a minimum shutter speed without IS, with camera hand-held, it's now a bit out-of-date because it used to apply to 8"x10" prints in the days when 35mm film was not considered good enough for prints much larger than 8x10".
In other words, if one uses an 80mm lens with FF 35mm, a shutter speed of 1/80th should be sufficient to overcome camera shake. However, to shoot the same scene with a Canon cropped format, one would use a 50mm lens. The question thus arises, does the 1/FL rule, as applied to APS-C format, mean that one can use a slower shutter speed of 1/50th?
No it doesn't. The 1/FL rule is really a 1/FL(35mm) rule. Whatever the format, the minimum shutter speed needed to overcome camera shake is '1/equivalent 35mm lens in millimetres', and that I guess would apply even to an 8x10 large format field camera unless there are some other issues relating to the difficulty of holding the camera. In other words, for a reasonably sharp 8x10" print, one could get away with 1/50th sec shutter speed, despite the fact that the standard lens on 8x10 format might be 320mm-400mm.
I think the confusion probably arises because people unwittingly sometimes compare apples with oranges. If the FoV is the same, if the print size is the same and if the shutter speed is the same, then the camera shake will be the same whatever the format, or to put it more precisely, the camera shake will be influenced only by one's ability to hold the camera steady. It might well be the case that a P&S digicam with LiveView LCD screen, held at arms length, will need a faster shutter speed to overcome camera shake, especially if one suffers from Parkinson's.