These days, employers are calling more references and questioning them more thoroughly than ever before, said Mark Oldman, founder of Vault.com, an online source of career information. Employers make those calls to ensure that applicants are not misrepresenting themselves and because they realize that "a bad hire can cost them more than waiting for the right candidate to come along," he said. That is why it is so important to choose references carefully. Contact them in advance, and make sure that they are prepared to speak favorably and in detail about your performance. If you do not lay that groundwork, you may be surprised at what your references are saying about you - if you ever find out.
See "The Workplace: Providing references", Phyllis Korkki, The New York Times, September 9, 2007