Time Capsule Multiple Backups: Both Encrypted? Got new Time Capsule. Got new iMac (OS 10.8). Hooked up Time Capsule and enabled encrypted iMac backup. 

Next, to my (little bit older) MacBook Air (OS 10.7). Trying to enable encrypted backup, but "Encrypt My Backup" checkbox is disabled (greyed-out).

Opinion differs. Some say MacBook Air backup is encrypted too, because "entire volume" of Time Capsule is encrypted by iMac. 

Not convinced. No user feedback on that from MacBook perspective.

What is the answer?
Encrypted backups only work when the backup disc is connected via USB.

It seems that there is a workaround for it using sparse bundle:
Mac OSX Lion Secure Backup to Time Capsule