Released on December 31, 2008, the new edition (the 4th) of the PMBOK has been updated and revised.
IMPORTANT: The PMP exam will change to the 4th Edition as of July 1, 2009. The CAPM exam follows shortly on August 1, 2009.
Passionate PM recommends that candidates for either exam do not use 3rd Edition materials as the sole source to study for the 4th Edition exam.
At first glance, it may not appear that the changes are significant. In fact, I've even seen the changes called "cosmetic". However, when it comes to the exams, the changes may be more than cosmetic in nature. Studying PMBOK 4th Edition is pretty much like reading the PMBOK for the first time!
The processes (now 42 of them) have had changes to their Inputs, Tools & Techniques, and Outputs (ITTOs).
Summary of Changes
The process names are now consistently in a verb-noun format (i.e. Define Activities)