Richard Rutter has a nice tutorial on sizing text with ems instead of using pixel based text sizes.
If the world were an ideal place, we’d all use pixels. But it’s not, we have the broken browser to contend with. IE/Win will not allow readers to resize text that has been sized in pixels. Like it or not, your readers will want to resize text at some point. Perhaps they are short-sighted, doing a presentation, using a ridiculously high resolution laptop or simply have tired eyes. So unless you know (not think) your audience won’t be using IE/Win or will never wish to resize their text then pixels are not yet a viable solution.
A good read, and I wish more people would do this (I'm looking at YOU safari). I think the thing that got me using Firefox full time was the fact that you can resize text, even if it was pixel based (well, just about every browser besides crap ass IE lets you do that). I mean, I sit there reading text of the screen for two to three hours. I'd just rather do that at 14px than 9px. Anyway, a good chunk of people can't make the the upgrade to Firefox, or simply don't know it even exists. So take pity on these poor souls, and at least let them resize your font if they need to.