[JoshTriplett]

Unfortunately, devices stopped actually making "mm" a millimeter long a while ago; for instance, various mobile devices scale "mm" by a factor of 2 because they assume that sites don't actually want a physical unit of measure.

[kickingvegas]

If you don't mind my asking: which devices?

[JoshTriplett]

Take a look at http://robert.ocallahan.org/2010/08/css-units-changes-landed... for some of the details.