Giving Information on a Former Employee

I don't know of any federal law that relates to your question. There are state laws regarding information an employer can disclose on an employee. In WA State, employers I know will only ask a previous employer the following:
1) Dates of employment
2) Whether they would hire that person again

"A previous employer is free to provide any non-confidential information about a previous employee, so long as it's true and isn't provided to maliciously harm the employee. An employer who provides false information that disparages the employee may be liable for defamation. In order to avoid potential liability, many employers often refuse to comment on a past employee's job performance and confirm only dates of hire and separation, plus wage or salary information."
source: http://research.lawyers.com/Washington/Employment-Law-in-Washington-State.html

In most states, yes, the prospective employer must obtain the written consent to applicant, and provide a copy of that to the former employer. In many states, simple information such as dates of employment, amount of compensation, and job title are all that can be provided. I managed several small businesses and this sort of thing came up frequently enough to learn that you need to check with each state, and that it is governed by the laws of the state in which the person applies for work.