In a blog post, Facebook Chief Privacy Officer Erin Egan said (emphasis ours),
"As a user, you shouldn’t be forced to share your private information and communications just to get a job. And as the friend of a user, you shouldn’t have to worry that your private information or communications will be revealed to someone you don’t know and didn’t intend to share with just because that user is looking for a job. That’s why we’ve made it a violation of Facebook’s Statement of Rights and Responsibilities to share or solicit a Facebook password.
"We don’t think employers should be asking prospective employees to provide their passwords because we don’t think it’s right the thing to do. But it also may cause problems for the employers that they are not anticipating. For example, if an employer sees on Facebook that someone is a member of a protected group (e.g. over a certain age, etc.) that employer may open themselves up to claims of discrimination if they don’t hire that person. [...]
"Facebook takes your privacy seriously. We’ll take action to protect the privacy and security of our users, whether by engaging policymakers or, where appropriate, by initiating legal action, including by shutting down applications that abuse their privileges."
Facebook really doesn't have much choice in the matter. Its users value their privacy. If it turns out that this practice becomes so prevalent that users feel uncomfortable with the service, those who have been unemployed for a long time might feel so threatened by an interview that they might bail on the service --- or give up their password, neither of which is a good option.
That's a little on the extreme side, but in this economy, it's not out of the question.