I just noticed this myself. I can't remember if I entered a backup e-mail address when I signed up, or what that address was. I guess we just change the info at apple.com and hope this works.
UPDATE: I clicked on the .mac link from the apple.com page, and it still allows you to go into your .mac account and change your backup e-mail address. I guess once your e-mail expires with .mac, they'll start sending statements to this backup address.