David Raynes just came out with a great new plugin for MoveableType: "SubCategories" Get itHere

Code for the subcategories list on my main index page

<MTSubCategories  show_empty=\"0\">
<MTSubCatIsFirst><ul></MTSubCatIsFirst>
<li>
<a href=\"<$MTCategoryArchiveLink$>\"><$MTCategoryLabel$></a><br>
<MTSubCatsRecurse>
</li>
<MTSubCatIsLast></ul></MTSubCatIsLast>
</MTSubCategories>