I originally saw this on MXNA. Jehiah Czebotar came up with a pretty good solution for making submit buttons or input type="button" elements only be as wide as they need to be. His blog entry. I did a slight modification for inclusion in a style sheet, and all seemed to work fine.
Good stuff. This beats the heck of my way of just putting a style="width:XXpx;" on every submit button. It seems to work in IE 6, Firefox 1.5, Opera 8.53 and Netscape 8.0.1