Override Magento to handle PHP 5.3+ namespaces.

Mage_Core_Model_Config.php

<?php
/**
 * Retrieve class name by class group
 *
 * @param   string $groupType currently supported model, block, helper
 * @param   string $classId slash separated class identifier, ex. group/class
 * @param   string $groupRootNode optional config path for group config
 * @return  string
 *
 * @override Override to handle PHP namespaces
 */
public function getGroupedClassName($groupType, $classId, $groupRootNode=null)
{
    if (empty($groupRootNode)) {
        $groupRootNode = 'global/'.$groupType.'s';
    }

    $classArr = explode('/', trim($classId));
    $group = $classArr[0];
    $class = !empty($classArr[1]) ? $classArr[1] : null;

    if (isset($this->_classNameCache[$groupRootNode][$group][$class])) {
        return $this->_classNameCache[$groupRootNode][$group][$class];
    }

    $config = $this->_xml->global->{$groupType.'s'}->{$group};

    // First - check maybe the entity class was rewritten
    $className = null;
    if (isset($config->rewrite->$class)) {
        $className = (string)$config->rewrite->$class;
    } else {
        /**
         * Backwards compatibility for pre-MMDB extensions.
         * In MMDB release resource nodes <..._mysql4> were renamed to <..._resource>. So <deprecatedNode> is left
         * to keep name of previously used nodes, that still may be used by non-updated extensions.
         */
        if ($config->deprecatedNode) {
            $deprecatedNode = $config->deprecatedNode;
            $configOld = $this->_xml->global->{$groupType.'s'}->$deprecatedNode;
            if (isset($configOld->rewrite->$class)) {
                $className = (string) $configOld->rewrite->$class;
            }
        }
    }

    // Second - if entity is not rewritten then use class prefix to form class name
    if (empty($className)) {
        if (!empty($config)) {
            $className = $config->getClassName();
        }
        if (empty($className)) {
            $className = 'mage_'.$group.'_'.$groupType;
        }
        if (!empty($class)) {
            if(strpos($className, '\\') !== false) {
                $className .= '\\'.uc_words($class, '\\', '_');
            } else {
                $className .= '_'.$class;
            }
        }
        $className = uc_words($className);
    }

    $this->_classNameCache[$groupRootNode][$group][$class] = $className;
    return $className;
}

Mage_Core_Model_Layout.php

<?php
/**
 * Create block object instance based on block type
 *
 * @param string $block
 * @param array $attributes
 * @return Mage_Core_Block_Abstract
 *
 * @override Override to handle PHP namespaces
 */
protected function _getBlockInstance($block, array $attributes=array())
{
    if (is_string($block)) {
        if (strpos($block, '/')!==false) {
            if (!$block = Mage::getConfig()->getBlockClassName($block)) {
                Mage::throwException(Mage::helper('core')->__('Invalid block type: %s', $block));
            }
        }
        if (class_exists($block, false) || mageFindClassFile(str_replace('\\', DIRECTORY_SEPARATOR, $block))) {
            $block = new $block($attributes);
        }
    }
    if (!$block instanceof Mage_Core_Block_Abstract) {
        Mage::throwException(Mage::helper('core')->__('Invalid block type: %s', $block));
    }
    return $block;
}

Varien_Autoload.php

<?php
/**
 * Load class source code
 *
 * @param string $class
 *
 * @override Override to handle PHP namespaces
 */
public function autoload($class)
{
    if ($this->_collectClasses) {
        $this->_arrLoadedClasses[self::$_scope][] = $class;
    }
    if ($this->_isIncludePathDefined) {
        $classFile = $class;
    } else if (strpos($class, "\\") !== false) {
        $classFile = str_replace("\\", DIRECTORY_SEPARATOR, $class);
    } else {
        $classFile = str_replace(' ', DIRECTORY_SEPARATOR, ucwords(str_replace('_', ' ', $class)));
    }
    $classFile.= '.php';
    return include_once $classFile;
}