I have a buddy that works at Garmin and from what I understand, GPS receivers are a huge battery drain and they are also pretty big/heavy in relation to other sensors. If you look at GPS running watches for instance, even the small ones are pretty beefy and they only last up to 8 hours while active.

This is a neat device, but GPS would seal the deal for me. I've looked at Tagg (http://www.pettracker.com/) which has it, but it isn't waterproof which a must for my Lab who loves to swim.