The FAR Cost Principles Guide has now been updated through FAC 2005-52 (June 2011) and is available for download here. The previous update was through FAC 2005-41 (April 2010). This guide keeps track of all changes to cost principles since the FAR was first published in 1984. If there is ever a question over the allowability of costs under a particular contract, this is a great resource for determining the specific regulations in effect at the time of contract award.
A few FAR cost principles have not changed since 1984 (e.g. bad debt expense, alcoholic beverages) but most have. 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.