Article from HRTools.com:

How Concealed Weapon Laws Impact Your Business
By: Jennifer Leahy | Wednesday, November 10, 2010

John loves to hunt, is a card carrying member of the National Rifle Association (NRA), and spends about 40 hours each week in your employ. He has a concealed handgun license. And tells anyone who will listen about his “right to bear arms.”
Where do his rights end and yours begin? Can you prohibit John from bringing a weapon onto your premises?
Probably not.

Traditional bans on all on-premise weapons by employers are continuously challenged in the courts, and there have been several significant changes that impact employers. More appear to be on horizon.

In the recent McDonald v. City of Chicago ruling, the U.S. Supreme Court declared the Second Amendment applies to state and local governments through the 14th Amendment and that handgun bans by state and local governments are unconstitutional under any standard of judicial review.

Although in all likelihood the issue will continue to be debated in the courts, the current climate indicates that workers cannot be legally forbidden to possess weapons at the workplace.

The “castle doctrine” that allows an individual to protect their property has been interpreted to include “home vehicle and workplace,” says certified Personal Protection Specialist Joshua Nobles.

Under such an interpretation employees would be allowed to possess a weapon (including guns, knives, or other items that might inflict harm) inside a locked vehicle at their place of employment, he says.

Employers can, however, require employees to undergo a registration process for on-premise weapons, says corporate security expert Don Greenwood of Don Greenwood and Associates Inc.

Weapons in the workplace are an emotional and complex issue that can stymie even the savviest of entrepreneurs.
“Companies didn’t do weapons policies until the 1990s,” says Greenwood.

The myriad regulations at the local, state and federal level are confusing and sometimes contradictory.
“It’s a prohibition of a prohibition type of scenario,” offers Nobles.

Even though you may be opposed to weapons of any kind and vehemently insist that they are not allowed on the grounds of your business, such a policy opens up the possibility of a legal challenge by an employee who feels his Second Amendment rights are being violated.
Business owners who forbid all weapons on premises may also assume liability for the safety of employees on their way to and from work if the employee is not allowed to possess a weapon, Nobles adds.

“Right to carry” laws vary by state and employers should keep abreast of current legislation and judicial rulings in all states where they conduct business.