That's one of the topics covered in someone's [link|http://kmself.home.netcom.com/Linux/FAQs/backups.html|Linux Backups Mini-FAQ], under "Other Issues".
My thinking is that backups should be treated as data access violations waiting to happen, and that physical security over backups are essential.
The problems of encrypted data storage by default, and the inconvenience it mandates, has been covered broadly...though naturally I can't find any references to the issue immediately....