I just thought about it a bit more, and using it after unsubscribing to .Mac is possibly not going to work. That is, if your hard drive crashes and you need to reinstall on a new drive Backup will likely think it is on the trial again. That said you could still rip the files out by hand (IIRC it just stores them in a compressed folder.)
Hmmm... all right, I think I'll be looking into another backup option then, just to be safe. Perhaps SuperDuper?