My wife was in Boston with some friends from work for the Red Sox game yesterday, and was on the route congratulating runners as they ran the last mile or two for the finish. They didn't see, hear, or feel anything as it happened, and were able to get out of the city safely, if confusedly since the police weren't saying a whole lot as they were urging people off the streets and overtaxed cell & wireless service made it hard to get info as it happened. We also knew a guy who was running in the marathon, but luckily he had finished prior to the bombing and he and his family were out of harm's way by the time it happened.
These stories ended happily. Others did not, and my deepest sympathies go out to everyone affected in any way, whether that involves a loss of life, health, or even just a sense of safety.
I think we'd do well to keep the lessons taught to us by our friends across the pond at the time when this poster was more than just a clever decorating or easily-Photoshopped internet trend, but a mandate of open defiance to those who would try to keep them down. It's become a cliche to say "if we change, they (whomever they end up being) win," but some cliches have a whole lot of truth at the center of them. The perpetrators of these heinous acts will face justice, definitely in this world and hopefully in any others that may exist (depending on whatever cosmology you, they, and/or the universe subscribes to), but the biggest slap in the face we can give them is to go about our lives as unaltered as possible. We will not forget this, of course, but we will keep calm, and we will carry on.