Didn't see much evidence of knives when down there over Christmas. If you take a bus, put your lock-back in your checked luggage. If you carry, do so discreetly and you shouldn't have any problem. After all, campesinos carry machetes openly. There are a lot of cops and soliders on the streets of Mexico City (and elsewhere), and they carry submachine guns. Lots of security guards around, too. I noticed one in a restaurant who was wearing a Colt Python -- pretty expensive for a low wage position. They did frisk some passengers (not norteamericanos) getting on and occasionally the bus would stop somewhere, a uniformed person would get aboard and one or two passengers would be searched. Don't know what criteria they used. Didn't see any knives for sale except some tourist junk, probably made in Pakistan. The only person I saw using a knife outside a restaurant was a guy in Vera Cruz, cutting bait out on a wharf. He was using what looked like a common seaman's knife from 100 years ago.