Correlation is not causation. Several people have pointed out that crime is down and that gun ownership is up.
The thing is, you would have to find enough people who would have committed a violent crime but were a) worried they'd get shot in the process so decided to do something more constructive and b) were willing to admit to it to have any sort of proof that was the reason. I think it has something to do with it, but I'm not as confident that's a key reason to purchase firearms.
Crime overall has been declining for a while as it is. Some of it real, some of it 'statistical trickery' (as in, if we redefine crime we can say we're doing something without really doing anything). I think the more we can get people to start accepting that the responsibility for their safety starts with themselves, we'll see that drop even more regardless of gun ownership.
What this unequivocally proves though is that the Chicken Little cries from the Brady Campaign, VPC, and Dick "I'll shove this gun up your butt" Daley are meritless. We've been pouring millions of new guns into the hands of the populace and violent crime goes down. "More guns = More crime" has been proven false.
In science, this is the point where you look at your hypothesis and go "Well… shit… Missed that one, didn't I?"
But I'm sure the Brady Campaign will either take credit for the decrease and / or have a massive fundraising campaign that will bring in several dollars.