By default, the checkout link is returned by a getCheckoutUrl()class function Mage_Checkout_Block_Onepage_Link. If it is pretty simple:
public function getCheckoutUrl()
{
return $this->getUrl('checkout/onepage', array('_secure'=>true));
}
Third party extensions most likely override this class (I checked OneStepCheckout 1.4 and it works like that).
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