Time machine encrypted backups
The new FileVault is a whole drive encryption, so Time Machine doesn't do any sort of encryption. The kernel / OS handles encryption as the data gets written to the drive so it's an all-or-none type of thing.

Once encryption is on for a specific drive, all new writes are encrypted whether or not Time Machine or another process is writing. The "Encrypting..." status is transient where the system is going through every block on the device and encrypting it so short of disabling the entire drive's setting to be under FileVault control there is no way to opt out of encryption for some new writes while leaving the encryption enabled.