Some of my friends just recently started despectacle.com offering a microSD card adapter called "CryptSD" which would do pretty much what you are looking for. Though the encryption is not done by the camera, but the SD card adapter itself.

Not quite sure when it is going to be officially launched(..)

Whenever the camera is turned on, a new session key is generated to encrypt the pictures. This session key is held in memory as well as saved onto the sd card (encrypted, using the public key). As long as the sd card is still powered, you will be able to encrypte & decrypt the pictures you have taken. Once the sd card is turned of (aka. camera off), you will no longer be able to read the pictures you have already stored on the sd cad. Ofcourse, you are still able to access them from your computer by entering your PW.