I think relative font-sizing with "em" and "rem" [1] is a popular and good choice these days. "em" has been popular since 2004 after an article by Rutter [2].
IE9- (and Android 1.6-) didn't scale/zoom well when the font was set in px. But using px was widespread despite these (and other) cons because it was easy, reliable and consistent to implement across browsers. [3]
Pt (point type) are only for printer style sheets [4], not screen style sheets. Using them for screen styles is not-done and doesn't make much sense. [5]
That wouldn't help. Without wanting to go too deep into OT-territory:
Even if the DPI could be known (and afaik normally it isn't, it is just set to a value) and it would be possible to convert in into real inches (btw, I have no idea how big that is in real units) to be displayed on a monitor, how would a designer know how big any element, even text, should be? It depends on the available space, which depends on the size of the monitor/the size of the window.
To think about how absurd that would be, try to find a situation where someone would say "this heading should be 2 cm tall".
px as size lead to the same problem with the available space, but they at least correspond to the resolution.
IE9- (and Android 1.6-) didn't scale/zoom well when the font was set in px. But using px was widespread despite these (and other) cons because it was easy, reliable and consistent to implement across browsers. [3]
Pt (point type) are only for printer style sheets [4], not screen style sheets. Using them for screen styles is not-done and doesn't make much sense. [5]
[1] http://snook.ca/archives/html_and_css/font-size-with-rem
[2] http://clagnut.com/blog/348/
[3] http://alistapart.com/article/howtosizetextincss
[4] http://css-tricks.com/css-font-size/
[5] http://style.cleverchimp.com/font_size/points/font_wars.GIF