Since the links are not block-level elements, the properties that should make them center don't apply. It is possible to fix that nicely with this:
Code:
A.category-top, A.category-top:visited {
display: block;
text-align: center;
margin-bottom: -12px;
}
but unfortunately IE doesn't know how to handle the negative margin and intermittently erases a strip of the element below it.
So the best I can suggest without getting into editing the code in tpl_categories.php is this:
Code:
A.category-top, A.category-top:visited {
margin-right:22px;
}
which puts them both quite near the center.