I've had a lot of fun with the Zoombak GPS device. Ever wonder what's inside one of those little guys? I did, but couldn't find any images on the internet to satisfy my curiosity. So I dissected mine.
It uses a 3.7V 890mAh battery, a Siemens 133851-V02 cellular chipset, and a Cirocomm 574B GPS module. As suspected, you can clearly see the holder for the T-Mobile SIM card (which I've already removed).