I believe the case you're referring to was Triangle Rent-A-Car out of North Carolina using GPS tracking to ding a renter for excessive speeding.
IIRC, he refused to pay & they backed down after seriously bad PR in the national news.
As for others, I use National exclusively by company contract. Having rented cars for over four months out of the year all across the US & Canada, I have never received any notification of their ability to track speed or location. (Many times in 3 digit speeds)
*Opinion*
I would bet that only GPS mapping or onstar equipped cars could be tracked. These companies have a sh*tload of cars that they get for cheap from the automakers, I doubt they're going to spend any extra $$ for tracking capabilities given the low percentage of non-returns.