I was in Laredo a few weeks ago and walked across. While I was having a beer on the way out of town (Coronas for two bucks) a guy named Johnny sat with me and we talked for a bit. Nice guy, clearly a businessman, and stood out as being somewhat more cosmopolitan in his dealings with tourists in that he was clean cut, spoke well, and seemed to be a reputable "businessman." Clearly not one of the shady characters who walk up to you with a couple of skanks in tow. Said he had been doing it for 28 years. He filled me in on the area. He said that Boys Town is no longer in operation because it had become too dangerous, but I can't confirm that. He made a phone call while we were sitting there and a few minutes later a georgeous lady showed up, which he offered me. I declined with thanks and a maybe some other time. When I was younger it would have been a different story.
While I was walking around the area there were very serious soldiers armed with automatic weapons everywhere. I took a picture of a group of them with my phone. One of them spotted me taking the picture even though I was being discreet. He broke away, approached me, and made me erase the picture and that was that. I guess it boils down to your comfort level in such situations.
All in all, I wouldn't be concerned with crossing, but I would recommend that you smile and relax. You will get the same in return. Like anywhere else - if it's trouble you want you will find it. Stay on the main street and only go a couple of blocks up. You will see everything you might want to see and you can get a bite to eat and a couple of beers. There is a nice park/gathering place a couple of blocks up. I sat with a couple of old guys. Nobody said a word, we just enjoyed the day. I crossed over a year ago and there were no soldiers then; only an armored vehicle at the border. I had a much more relaxed experience this time, probably because I relaxed. However, let there be no doubt; the Los Zetas cartel, which controls the drug trade in Nuevo Laredo, has everyone there on edge. You can feel it. But the folks there are making the best they can of it and are keepin on.