About 1 years ago, when fixing font size in px in css, changing letter size in IE wouldn't change anything...
I discovered that, under IE 6 SP2, it's no longer the case!
They must have changed this behavior in one of the SP x ?!
I can't find a link in google about it! (the best I can get is Standards and CSS in IE and W3C spec)
Now I can use px with no more accessibility problems :)