it's common for sophisticated car thieves going after luxury vehicles to use GPS/cell phone jammers. there are some products using proprietary technology not reliant on a GPRS/3G signal.
For anyone looking for a DIY type thing, you can buy modems that use the Iridium satellite network for maybe $250. Line fees are about $12/mo and from what I remember when I looked, it's actually quite cheap if you just want to transmit short textual data (like location status updates). Worked out a lot cheaper monthly than using 3G from most of the carriers around here.
Of course, it comes with its own downsides. For instance you'll lose reporting if they pull in an underground garage. On the other hand, if they ship your vehicle to the South Pole you'll continue to have reception.
Of course, it comes with its own downsides. For instance you'll lose reporting if they pull in an underground garage. On the other hand, if they ship your vehicle to the South Pole you'll continue to have reception.