I stumbled across this set of css properties which seem to vertically align the text in sized select elements in Firefox:
select
{
box-sizing: content-box;
-moz-box-sizing:content-box;
-webkit-box-sizing:content-box;
}
If anything, though, it pushes the text down even farther in IE8. If I set the box-sizing property back to border-box, it at least doesn't make IE8 any worse (and FF still applies the -moz-box-sizing property). It would be nice to find a solution for IE, but I'm not holding my breath.
Edit: Nix this. It doesn't work after testing. For anyone interested, though, the problem seems to stem from built-in styles in FF's forms.css file affecting input and select elements. The property in question is line-height:normal !important. It cannot be overridden. I've tried. I discovered that if I delete the built-in property in Firebug I get a select element with reasonably vertically-centered text.