As Mountain Lion is coming this month, I would like to do a clean installation of my dual-boot iMac, but as I use more Ubuntu, I want to give it more HD space than it haves now (125 GB in a 500 GB HDD, the rest of the HD is used by Mac OS). I have some backups done with Déjà Dup in an external HD, and I saw that they're done using GPG. Is then possible to restore my backup after formatting the hard disk?
Yes, it's possible. Just do a backup before formatting the computer, and, after installing Ubuntu, open Déjà Dup and follow the instructions. Tested myself!