I wonder how these high-res tablets would affect HTML5 media queries. Right now, many of us do something like "screen and (max-device-width: 480px)". That's going to get rather awkward if pixel density varies a lot between devices. We won't be able to fool ourselves forever by implicitly multiplying that by 2 (or some other factor).
Perhaps we'll need to standardize on a unit that is independent of screen resolution. But it won't do to convert everything to millimeters, because different devices are also meant to be used from different distances. 2mm on a smartphone is OK. 2mm on a cinema display is virtually invisible.
A CSS px is not necessarily defined as a pixel -- The reference pixel defined in the CSS spec is the length crossed by a 0.0213 degree arc at the typical viewing distance (which works out as 96 DPI at arm's length).
This was solved five years ago: http://www.webkit.org/blog/55/high-dpi-web-sites/ Summary: px is a logical unit and 1 px = 2 (or more) pixels on high-DPI displays. Em is also a logical unit, although I think there are problems with em-based layout.
Perhaps we'll need to standardize on a unit that is independent of screen resolution. But it won't do to convert everything to millimeters, because different devices are also meant to be used from different distances. 2mm on a smartphone is OK. 2mm on a cinema display is virtually invisible.