While reexamining the way we handle screen resolutions in Gecko, we had to reconsider how to handle CSS "absolute length units" --- especially pt, but also in, cm, mm and pc. The spec says that these units should be displayed at their physical sizes, so CSS "1in" is rendered as one inch, etc. Unfortunately this breaks Web content, because most Web pages were designed on desktop screens. A one-inch margin may be fine on paper or on a desktop screen but doesn't make much sense on a two-inch-high phone screen. For related reasons, IE and Webkit have already redefined these units to be fixed numbers of CSS pixels, i.e. 1in = 96px, etc.
After I raised this issue on www-style, it became clear that this is not just an issue of compatibility with existing content. In fact, absolute length units as defined in CSS are only rarely useful. When do you want to force a length to be one inch, no matter what kind of screen is being used or how far from the user's eyes it would be? The only use cases I know of would be touch interfaces and "life size" diagrams. Indeed, the CSS spec says
Absolute length units are only useful when the physical properties of the output medium are known.
On the Web, physical properties of the output medium are almost never known, so the spec suggests that currently absolute length units are useless on the Web.
It seems to me that it will be far more useful, as well as compatible with existing Web content, to specify that absolute length units should take those physical lengths when printed to "normal paper" --- paper (or other media) that you hold when reading, like almost every printed page. Then the browser decides how to render the content on a screen to best express the author's intent. Common sense, as well as Web compatibility, will dictate that the ratios between "px", "pt", "in", "cm", "mm" and "pc" will be fixed based on the assumption that 1in = 96px.
We may find it useful to define a new unit or units, say "truemm", that is a true physical millimeter, for the rare use cases of touch interfaces and "life size" diagrams.
Redefining "in" to not always mean physical inches is quite controversial in www-style. I hope sanity prevails.