require_once 'Mage/Checkout/controllers/OnepageController.php';
class MarlowWhite_Checkoutmods_OnepageController extends Mage_Checkout_OnepageController
{
    public function indexAction()
    {
		print "hi";die();
        parent::indexAction();
    }
}
<config>

	<frontend>
		<routers>
			<checkoutmods>
				<use>standard</use>
				<args>
					<module>MarlowWhite_Checkoutmods</module>
					<frontName>contentcart</frontName>
				</args>
			</checkoutmods>
			<checkout>
				<use>standard</use>
				<args>
					<modules>
						<checkoutmods_marlowwhite before="Mage_Checkout">MarlowWhite_Checkoutmods</checkoutmods_marlowwhite>
					</modules>
				</args>
			</checkout>
		</routers>
	</frontend>
....
</config>