This information has been updated here.
The FAR Cost Principles Guide has been updated through FAC 2005-41 (April 2010) and is available for download here. This guide keeps track of all changes to cost principles since the FAR was first published in 1984. If there is ever a dispute over the allowability of costs under your contract(s), this is a great resource for determining the specific regulations in effect at the time of contract award.
Some FAR cost principles have not changed since 1984 (bad debt expense). Other principles have changed many times. For example, the Compensation cost principle which covers the allowability of compensation costs has changed 30 times. That averages to more than once a year.