This is just the beginning. If insurance companies in this field - who are more risk adverse than most (I know, I'm a retired minister) - are now looking at the increased risk of banning guns, what do you think will happen after the first business looses a lawsuit after their "no guns allowed signs" attract a mass shooter?
In fact, I would be shocked if attornies suing the Colorado theater victims are not planning to use that very argument at trial. If they do, and they win, you can look for insurance companies to start refusing to insure business that are "posted". This will result in huge numbers of these signs coming down.
If this happens, and I think at some point it will, look for the antis to demand that business posting signs be exempted from the liability of creating "phony gun free zones". Our counter should be that since "phony gun free zones" are dangerous - they should be banned, or at least warning signs should be required indicating that "Places where guns are banned incur an increased chance of being killed." We should stress that "gun free zones" are are not gun free, and that if a business wants to be "gun free" they need to crease a real gun free zone - as in airport like security. We need to stress the absolute absurdity of thinking that anyone with evil intent is going to be stopped by a sign. I think that if they try to get liability protection, we will win - and if they don't, we will win.