I’m guessing you mean using ems rather than px when sizing text?
Ems are based on percentages, which means they can increase/decrease in size if the user changes his default font size in the browser. Unfortunately, IE will not allow readers to resize text that has been sized in pixels. Basically, it’s an accessibility thing — ideally, the visitor should be able to increase/decrease their text size if they choose.
More info: http://www.clagnut.com/blog/348/