Just FYI, this is what the CMUcam2 manual has to say about the idle command:
"This command is used to set the camera board into an idle state. Like all other commands, you should receive the acknowledgment string “ACK” or the not acknowledge string “NCK” on failure. After acknowledging the idle command the camera board waits for further commands, which is shown by the ‘:’ prompt. While in this idle state a \r by itself will return an “ACK” followed by \r and : character prompt. This is how you stop the camera while in streaming mode."
-Kevin