em or relative font-sizing seems like a good choice. Also see rem [1] which was introduced with CSS3 to combat issues with font size compounding. Use pt for print stylesheets [2].
The rem unit is awesome! All the benefits of em (relative to browser text zoom level) combined with the benefits of px (does not change depending on parent element font size).
If you don't need to support IE8 you should start using rem today.
For a bit of ancient internet history: http://style.cleverchimp.com/font_size/points/font_wars.GIF
[1] http://snook.ca/archives/html_and_css/font-size-with-rem
[2] http://www.w3.org/Style/Examples/007/units.en.html