I have a 2-year old PowerMac G4 with a DVD-Ram, and Backup refuses to let me use it. Backup will also limit the online option to .Mac iDisk only, which means you can't backup to another FTP or WebDAV-enabled server.
Keep in mind those limitations if, besides keeping your @mac.com email address, you were considering Backup as another reason to throw $50 for .Mac. This application, while very easy to use, does not compare well with other backup products.
Apple simply needs to unlock a few things and it will become another great application. Meanwhile, as the author of this article, you'll probably need to keep your current backup solution.