Gun free zones seem to get a lot of hype. It's my understanding that, at least in Minnesota, it's a petty misdemeanor to carry a gun into any private business that doesn't allow guns - IF the establishment asks you to leave and you refuse.
I would bet those who understand the law and keep things concealed ignore those signs all the time. I know I would.
http://thompsonhall.com/minnesota-gu...stablishments/