<global>
    <events>
      <controller_action_predispatch_checkout_cart_index>
        <observers>
          <redirect_to_checkout_from_cart>
            <type>singleton</type>
            <class>[ModuleNameSpace]_[ModuleName]_Model_Observer</class>
            <method>forceToGoCheckoutPage</method>
          </redirect_to_checkout_from_cart>
        </observers>
      </controller_action_predispatch_checkout_cart_index>
    </events>
  </global>

<?php
class ModuleNameSpace]_[ModuleName]_Model_Observer{
    public function forceToGoCheckoutPage($observer){
        // force redirct to checkout page
          Mage::app()->getFrontController()->getResponse()
                ->setRedirect(Mage::getUrl('chekout/onepage'))
                ->sendResponse();
exit();

    }
}

public function forceCheckoutRedirect($observer){

  $numberOfItems = Mage::helper('checkout/cart')->getItemsCount();
  // force redirct to checkout page if cart is not empty
  if($numberOfItems > 0) {
    Mage::app()->getFrontController()->getResponse()
    ->setRedirect(Mage::getUrl('onepage'))
    ->sendResponse();
    exit();
  }
  else {
    Mage::app()->getFrontController()->getResponse()
    ->setRedirect(Mage::getUrl('/'))
    ->sendResponse();
    exit();

  }

}