I would like to use my Ubuntu One account to back up some files with Déjà Dup. Is this possible?
Yes, Déjà Dup now (as of Ubuntu 11.10) supports backing up files directly into Ubuntu One.
Select 'Ubuntu One' under 'Storage', like below:
You should be able to sync your back ups with Ubuntu One by setting your backup location to your Ubuntu One folder in your Home folder.