Pixels don't mean much to ordinary users.
Use meters/inches instead. This can be done by obtaining the monitor DPI...

This is relatively easy to do using gdk_screen_width_mm().

Now displays in meters.

I'm not sure I agree here.
Meters don't mean much either, since these are meters the mouse pointer moves
over the screen. The actual value of interest would be the number of meters you
actually move your mouse. This also depends on the mouses speed and acceleration
settings.

So I would argue that pixels mean exactly as much as meters: It's some value
that is proportional to the amount of distance the mouse actually travelled
(assuming acceleration is turned of). Hence, I would be in favor of displaying
mouse distance in pixels, since this makes it very clear that we are not talking
about the distance the mouse travelled, only the mouse pointer.

We're dealing with ordinary users here. For them, meters really mean more then
pixels because they can relate that to something that has a meaning in real
life. For example, my girlfriend (err, wife) does not know how many pixels a
screen is. For her, 143214 pixels per day has no meaning at all, none
whatsoever. Meters she can at least relate to, even if it is only about screen
meters instead of mousepad meters.