My bad, I just discovered that text-indent on input elements causes problems in older versions of Internet Explorer. Perhaps that is why Gideon used the font-size and colour trick. That is still problematic though since some browsers (Opera and Firefox, at least) have a minimum font size so 1px will always display at about 8px or higher.
My current solution is to scrap all of those and blank the VALUE attribute on the buttons. I'm also converting them to display:block in my own code for more reliable sizing/positioning. Will update my post shortly...