Human Resource departments responsibilities for security have increased in relation to 9/11 and in relation to identity theft, and these two areas of security are related to each other. In verifying identity and personal information, checks are also run for terrorist alerts and for bank account irregularity alerts, since these alerts are automatically generated while doing background, bank, and credit checks. Human Resources must spot and prevent identity fraud, background fraud and potential security breaches. An example of Human Resources goals and objectives clearly distinguishes these several responsibilities. These are relatively newly developed and are more specialized since 9/11.
My first interpretation of this question relates to hiring practices and background checks on potential employees. Human Resources is often responisible for hiring and therefore vetting candidates for employment. Though I don't think the responsibility to thoroughly vet potential employees has changed, there may be a greater emphasis in HR departments on assessing emotional stability, personal history, and other factors relating to safety.
Actually, I do think companies are responsible for keeping employees safe. This is especially true in the case of valuable or important materials being held at the company. If your company is a target, you need to keep them safe. That part might not have changed since 9/11, but now we can add that if your company is in a government building or other target you have an extra duty to protect employees.
Human Resources departments have improved security, although not perhaps in their hiring as much as in developing more specific procedures and protocols for protecting employees in the work place. Since September 11th, many corporations have put procedures into place for employee lockdowns and evacuations. Moreover, businesses, especially those in large buildings want to convey an atmosphere of security and safety to potential climates as well as employees. Security has become about visibility; self-locking doors, checkpoints, required badges have all become more common as human resource departments try to instill a visible presence of security among employees and customers.
For the most part, I do not think that it has. The burden of providing security is still much more in the hands of the government than in the hands of HR departments in the private sector. Of course, concerns about things like industrial espionage exist, but these are not new.
Perhaps the clearest indication that security is not the province of HR departments comes from the issue of illegal immigration. Illegal immigration is of concern in part because terrorists could use the same routes into the country that illegal immigrants do. This being the case, HR departments are still not very vigilant about trying to weed out illegal immigrants. HR departments are much more concerned with the economic strength of their firms than with security.