Cars already have data logging black boxes, and, in the case of OnStar, can already send out a help message in a crash and record in-car audio. Connecting that with the in car GPS is only a software program away.
Do you think they'll become a)required, and b) used for legal purposes, either post-collision or traffic law enforcement?
To the first question: Yes, they will eventually become required or at least standard equipment on nearly every vehicle (even though most vehicles already have them).
How will this happen you ask? Through the Insurance Institue for Highway Safety. The black box saves insurance companies money. Therefore, the IIHS will see it as a "safety item" and rate cars poorly that don't have it. Eventually the NHTSA (the goverment version of the IIHS) will follow like they always do and we'll see black boxes becoming mandatory or at least standard equipment. It's already happening. This is how Tire Pressure Monitering and Stability Control recently became required features.
To answer your second question:
http://www.law.com/jsp/article.jsp?id=1105364095740