Updating computed field's value without needing to clear cache
Check out the rules module (http://drupal.org/project/rules):


  The Rules module allows site administrators to define conditionally
  executed actions based on occurring events (known as reactive or ECA
  rules).


You could add a rule to clear the cache whenever a new nodes has been added. 

It's not exactly the way you asked for, but it gets the job done - updating the computed field's value whenever a new node has been created.