The CSS units changes that I blogged about in January have landed for beta 4.
With these changes, 1in = 96px always. Likewise 3pt = 4px, 25.4mm = 96px, etc. This matches the behaviour of Internet Explorer, Safari and Chrome.
By default, when printing, 1in is rendered as one physical inch. For other output media, all these units are scaled in a medium-dependent and platform-dependent way by default. One goal of this scaling is to give results consistent with user expectations and other applications on the system. For example, standard form controls such as checkboxes should look the same in Web pages as in other applications, by default. Another goal is to choose default scaling so that a document designed to print well on normal-sized paper will be readable on the output device, e.g., a phone.
There are some uncommon cases where it makes sense to include true physical measurements in a Web document --- for example, "life size" diagrams, or elements in a touch interface. For these cases we have introduced a new experimental unit, "mozmm". For media such as screens that can be touched, 1mozmm is rendered as one physical millimetre (or as close as we can get based on what we know about the medium). For other media, such as contact lens displays, brain-implanted electrodes, or lasers projecting into the sky, we reserve the right to treat 'mozmm' similarly to 'mm'.