Symfony2 dev environment not clearing cache automatically

I'm working on a new Symfony2 project, and I'm using the app_dev frontcontroller to view the default AcmeDemoBundle's pages.

When I make changes to the controllers, I get an exception (NotFoundHttpException: No route found for "GET /demo/hello/World"). If I clear the cache using php a开发者_开发问答pp/console cache:clear, things work great again. However, it is my understanding that by using the app_dev front controller, I shouldn't be required to do this. Am I doing something wrong?

Soms system specs: OSX Lion using MAMP PHP 5.3.6 Symfony 2.0.1

Are you using different users for webserver and command line user? Maybe you should check for permissions in the Setting up Permissions paragraph.