The box does exist in all post 96 cars. The problem is that, like mentioned above, it only records a few seconds at a time to track engine/emission performace. Can speed, rpm, brakign be calculated with this info? - yes. But right now its protected by some law, but be warned, the NJ state police are challenging it so they can have access for "forensic purposes."
Data recorders are not to worry about though as they are tough to use against you in court. If you want scary take a hard look at OnStar.
The joke above is based in fact. In 2002 OnStar randomly selected and recorded the events inside several hundred thousand cars. The project was granted under the guise that "it was to monitor drivers reactions to events" and since it didn't keep names of the people being recorded GM side-stepped litigation. (oh, offically the most common phrase mutter before impact was the more colourful variant of "oh crap")
Onstar currently operates at ~40% its full capacity. At peak it can record all conversation, monitor all vehicle data (speed, engine, ect) AND locate through GPS EVERY SINGLE VEHICLE ITS CURRENTLY INSTALLED IN. GM is planning on using it to "remind" you to bring you car in the moment the check engine light comes on, less you rather not have a warrenty.
Want more paranoia - both GM and Benz are suggesting making OnStar standard equipment on all cars they produce, even if you don't subscribe...