- Delete your current .mac account from the Preferences Pane.
- Recreate the account, but choose IMAP instead of .Mac for the account type.
- In the email address field, add your email address plus all the email aliases you have separated by commas.
[robg adds: Take care when you delete an account in Mail -- I'm paranoid of losing messages, so I always backup before doing anything like this. I have not tested this one yet, but will do so once I get around to setting up some aliases.]

