A List Apart has a good article on how to do relative font-sizes in your webapps.
I'll have to remember this the next time I need relative fonts in a webapp. I've just been using "font-size: 12px" on the
body and "em" for everything else. Sure resizing fonts doesn't work in IE, but no one has complained yet. And since I use Safari or Firebird/fox all the time, I never notice.