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Not quite Makten. An IR filter accentuates the red drift but the drift is because of the ray angle. Simply put, with film it isn't important at which angle the light hits the medium. With a digital sensor it does - a too steep angle and you get the color drift. Now the M9 sensor has been designed with this in mind with increasing angle of the photo sites toward the edges. What they could not do with that they have addressed with software (lens profiles).
As far as I know, the pixels on the M9 are normal, and only the micro-lenses are shifted progressively towards the middle as you near the edge and corners.
There are two kinds of IR filters, and the ones used in Leica's lens filters for the M8 are the interference type, as is the (too-thin) filter on the sensor. Several thin coatings are deposited with thicknesses around the wavelength of the undesirable wavelength of light, i.e. IR here. This cuts the IR very effectively, but towards the corners, as the light impinges at a greater angle, the distance between the layers of coatings are no longer the right size, and some IR is let through, hence the in-body corrections on the M8.
On the M9, they used an IR-absorbing filter on the sensor instead. This type is generally less effective, unless you can get it thick enough, but having a thick piece of glass in front of the sensor causes secondary (and tertiary...) reflections, leading to ghosts and lacking sharpness. Leica found a way for the M9 to keep the filter thin, but still make it effective. I expect another notch of improvement in the M10.