Thanks. I found some stuff at Heritage and elsewhere that claimed that U.S. trucks were allowed into Mexico as late as 2001, but then Vicente Fox threatened to close his border to our trucks if we didn't open ours to his. I cannot find anything that indicates the current state of this dispute. I know that it hasn't gone to the WTO, so it has either been resolved or is in the process.
As for the dispute, Mexico took the US into arbitration and was awarded 2 billion per year early in 2001.
Final Report of of the Arbitration Panel
Although Mexico is entitled to sanctioned retaliation to the tune of two billion per year, and it has been a hot topic in the Mexican Congress, they have as yet retaliated.