Graeme Mathieson has produced 
a great tutorial on using Mail.app as an offline archive for Gmail, complete with step-by-step instructions on creating the rules to push emails into the right folders and screenshots.
He recently switched to using Google apps for your domain but like any prudent Scot wanted access to his email when he is not online and “a backup should something go wrong” (see further — “Gmail user gets kneecapped” and the Gmail “Black Hole treatment”).
Nice.
