Display Top Level Categories and ALL Subcategories

<?php $_helper = Mage::helper('catalog/category') ?>

<?php $_categories = $_helper->getStoreCategories() ?>

<?php $currentCategory = Mage::registry('current_category') ?>

<?php if (count($_categories) > 0): ?>

    <ul>

        <?php foreach($_categories as $_category): ?>

            <li>

                <a href="<?php echo $_helper->getCategoryUrl($_category) ?>">

                    <?php echo $_category->getName() ?>

                </a>

                <?php $_category = Mage::getModel('catalog/category')->load($_category->getId()) ?>

                <?php $_subcategories = $_category->getChildrenCategories() ?>

                <?php if (count($_subcategories) > 0): ?>

                    <ul>

                        <?php foreach($_subcategories as $_subcategory): ?>

                            <li>

                                <a href="<?php echo $_helper->getCategoryUrl($_subcategory) ?>">

                                    <?php echo $_subcategory->getName() ?>

                                </a>

                            </li>

                        <?php endforeach; ?>

                    </ul>

                <?php endif; ?>

            </li>

        <?php endforeach; ?>

    </ul>

<?php endif; ?>