Ranger is definitely getting the idea that he can trade the left paw lift (food sign) for treats but I'm not sure that he recognizes it as a "word" or label for food items. We spent part of the day signing food and giving treats; Ranger signs with us or "fills in the blank" and signing toy and tossing his favorite squeaky toys. Ranger has yet to offer the sign for toy without prompting.
We went to our favorite pet store today and while one of the clerks was fishing in the bottom of the bag for the elusive last treats Ranger sat politely and signed food. I praised him and the clerk delivered the treat. He volunteered the "sign" but it was in the context of someone being slow to provide the treat he expected. I'm going to be more convinced that he understands it as a word when he comes in and asks for something to eat when there's no context other than him wanting to "tell" me something.