Most GPS devices do not transmit data. Put simply, GPS works by a constellation of satellites transmitting location data, which is triangulated by the GPS receiver. Thus, there is no need for GPS' to transmit data, only receive data.
If you want to design an automatic vehicle locator similar to cabulous you need a way to get the GPS coordinates from the GPS receive to some server.
Typically in the past this was done with specialised hardware, either using the mobile phone network or even proprietary radio networks. With the advent of internet-connected, GPS enabled smart-phones such as the iPhone or Nokia 5230, the hardware side is already done. All that is required is custom software to sit on the smart-phone, and a server somewhere to receive location data.
I don't know of any freely available services that might help you, but hopefully this will give you some idea of where to start.