Will the old OS 9 Retrospect client software still talk to the current OS X server version? That'd be a slick way to back up the old and the new to one Mac, then back up that data to a remote location. I've been planning a new backup plan and backing up to one computer, then backing that one up to some form of online storage as a redundant offsite solution for a worst-case scenario is what I've been kicking around.

Right now I don't have the need to to regular backups of the exact contents of my old Macs. I have the software installed on them archived elsewhere and could recreate the contents of any drive in the case of a failure. Your uses may vary and make regular backups more appealing. (Plus there's always geek cred...)