It seems that loudly pointing out the Mexican Gun Lie

has had an effect:
The "90%" claim has, in fact, been so thoroughly discredited that even those who tried to make political hay of it have been forced to back away from it. Here's Brady Campaign president Paul Helmke:

Rather than argue about percentages, let's focus on the fact that 20,000 trafficked guns from America have ended up at Mexican crime scenes.

We'll talk about that "20,000" figure in a minute--first, though, let's look at Helmke's casual willingness to not "argue about percentages."

He certainly seemed interested in arguing about them back in April:

As the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) reports, 90 percent of firearms recovered at Mexican crime scenes have been traced to gun sellers right here in the United States.

...
So, what about those "20,000 trafficked guns"--isn't that rather damning? Well, certainly it's nothing to be happy about. Oh--did I mention that that number represents 5 years worth of seized guns? Let's think about some context here. How many illegal immigrants have come across the border in the last 5 years? How many tons of illegal drugs? If your answer, in both cases, was "a heck of a lot more than 20,000," give yourself a cigar.
And, for that matter, let's keep pointing out the effectiveness of all the laws the Brady Gun Ban Group love:
The same report cited by Helmke and Sugarmann states that 4,000 of those trafficked guns came from California--#1 Brady Campaign ranked California, where nearly every "anti-trafficking" measure the Brady Campaign can think of (one gun per month, no "gun show loophole," waiting periods, registration of gun owners, state licensing for dealers, etc.--not to mention the strictest faux "assault weapon" ban in the nation and a ban on .50 caliber rifles) is already in place. California in fact, is apparently the second largest source state of guns trafficked into Mexico (Texas is first). Proximity to the border would seem to play a much larger role than the restrictiveness of gun laws.
I'm going to post that quote from The Weapon again:
I acquired a black-market weapon.(Sales are all registered, but for a small premium of 50%, one may buy a new pistol not far from the spaceport. They come in by the shipload, as they do anywhere the local rulers utter magic incantations to keep them out. Never let your religious beliefs get in the way of the law of supply and demand.)