I'm playing with the forwarding options on the Define Rules screen to try and make them more consistent (previously the "Forward message" option would really "redirect" the message as the original From address on the message was preserved, while the "Forward summary" option really did forward a summary, using your account name as the message From address).
If you login at the beta server (http://www.fastmail.fm/beta/) right now you'll find the following forwarding options are available:
You can interpret the main actions as:
And the parts to forward as:
This is a bit more orthogonal and keeps the terminology of "redirect" vs "forward" more consistent, including being consistent with the use of "Redirect to people" in the Action menu on the Mailbox screen (preserves the original From address), and the "Forward" button on the View message screen (creates a new message with your username as the From address)