If you want to load your cusom router prior to all other router in Magento, then try this one.

Define your custom router in `config.xml`
---------------------------------------

File : `app\code\{codePool}\{Namespace}\{Module}\etc\config.xml`

    <config>
    	<default>
    		<web>
                <routers>
                    <{router_identifier}>
                        <area>{admin|frontend}</area>
                        <class>Namespace_Module_Controller_Router</class>
                    </{router_identifier}>
                </routers>
            </web>
    	</default>
    	<stores>
            <default>
                <web>
                    <routers>
    	                <{router_identifier}>
    	                    <area>{admin|frontend}</area>
    	                    <class>Namespace_Module_Controller_Router</class>
    	                </{router_identifier}>
               	 	</routers>
                </web>
            </default>
        </stores>
    </config>

Now Make `core` module has dependency on your module
------------------------------------------------------

File : `app\etc\modules\Namespace_Module.xml`

    <?xml version="1.0"?>
    <config>
    	<modules>
    		<Namespace_Module>
    			<active>true</active>
    			<codePool>{local|community}</codePool>
    		</Namespace_Module>
    		<Mage_Core>
    			<depends>
    				<Namespace_Module />
    			</depends>
    		</Mage_Core>
    	</modules>
    </config>

Now clear your cache. You are done.

Why does This work
-------------------

Router loading is happening inside *front-controller*<sup>**1**</sup>. The abbreviated form of router loading code is shown below.

    public function init()
    {

    	//prioriy 1 : routers adding via xml configuration
        $routersInfo = Mage::app()->getStore()->getConfig(self::XML_STORE_ROUTERS_PATH);
        foreach ($routersInfo as $routerCode => $routerInfo) {
            if (isset($routerInfo['disabled']) && $routerInfo['disabled']) {
                continue;
            }
            if (isset($routerInfo['class'])) {
                $router = new $routerInfo['class'];
                if (isset($routerInfo['area'])) {
                    $router->collectRoutes($routerInfo['area'], $routerCode);
                }
                $this->addRouter($routerCode, $router);
            }
        }

        //priority 2: routers adding via observer
        Mage::dispatchEvent('controller_front_init_routers', array('front'=>$this));

        // pririry 3 : for default router.
        $default = new Mage_Core_Controller_Varien_Router_Default();
        $this->addRouter('default', $default);

        return $this;
    }

This method simply grabs routers from various resources and add it to the protected property `$_routers` as an array<sup>**2**</sup>. As you can see, the priorities are like this.

1. Routers which is defined via xml files will add to `$_routers` first.

2. Router which are added via the event `controller_front_init_routers` go next.

3. At last, Default router will be added to the array.

Now, *standard* router and *admin* router are defined in `app/code/core/Mage/Core/etc/config.xml` and hence they will be loaded first in `$_routers` array and thus they will be processed first.

In order to change this intentional priority for the standard and admin router, you need to define your router via your module's `config.xml` file. This will bring your router to the priority level 1. Making core module depend on your module gives more priority to your module and hence Magento will consider your module first and hence load your router in `$_routers` array first. 

Hope that makes sense.

------------------------
<sup>1: Mage_Core_Controller_Varien_Front::init()</sup>

<sup>2: `$this->addRouter()` is actually what populates `$_routers`.</sup>