I while ago I started putting together my own tracking package but never finished
it because my tablet has not been stolen yet. The basic premise was:

1. upon connection to internet, retrieve a specific file from my webserver. the
request may include nearby wifi router macs.

2. if the file retrieves ok, execute it

this is very simple and means i can do the rest of the implementation
later if needed.

the webserver logs the ip address and router macs. the macs' location can be
looked up (eg: on wigle.net).

normally, my webserver will just fail the request. if i lose my tablet, i
can place a suitable script on the server. an example script might set up a
tunnel back to my pc so that i can ssh into the tablet and do gps logs, delete
files, camera capture etc, even behind a firewall.