<?php
$cats = Mage::getModel('catalog/category')->load(2)->getChildren();
$catIds = explode(',',$cats);
?>
<ul>
<?php foreach($catIds as $catId): ?>
<li>
<?php
$category = Mage::getModel('catalog/category')->load($catId);
echo $category->getName();
$subCats = Mage::getModel('catalog/category')->load($category->getId())->getChildren();
$subCatIds = explode(',',$subCats);
?>
<?php if(count($subCatIds) > 1):?>
<ul>
<?php foreach($subCatIds as $subCat) :? >
<li>
<?php
$subCategory = Mage::getModel('catalog/category')->load($subCat);
echo $subCategory->getName();
?>
</li>
<?php endforeach;?>
</ul>
<?php endif; ?>
</li>
<?php endforeach; ?>
</ul>