I have the current Whistle activity monitors on my dogs and I have ordered the gps model. It won't be out until 2015. They just had a preorder special for a couple of weeks where it was $49. It might be bulky for a cat. I know there were questions about cats on their Facebook page. They explained that the gps feature would work but that the activity monitoring is geared to dog activity.
You can see it on Sam's collar in a photo I just posted in this thread: https://www.lab-retriever.net/board/f...-inspired.html