I’ve found the distinction between redirect() and forward() a little tricky to draw. It always seems to sound obvious until you need to use them. I found a nice article here http://firebird84vn.wordpress.com/category/symfony/. To quote:

The choice between a redirect or a forward is sometimes tricky. To choose the best solution, keep in mind that a forward is internal to the application and transparent to the user. As far as the user is concerned, the displayed URL is the same as the one requested. In contrast, a redirect is a message to the user’s browser, involving a new request from it and a change in the final resulting URL.

If the action is called from a submitted form with method="post", you should always do a redirect. The main advantage is that if the user refreshes the resulting page, the form will not be submitted again; in addition, the back button works as expected by displaying the form and not an alert asking the user if he wants to resubmit a POST request.

OK, so basically a submitted form should be redirected by the action (as opposed to doing a forward). In other situations where you’re calling part of your own application, just use a forward(). Maybe I’ve missed something, but this seems pretty straightforward (pun unintended) after all.