Eric,
Generally the Project Management Plan is updated when there are significant changes to how you are going to execute the project. This typically comes in the form of updates to the components of the Plan, such as when you adjust your Cost Management Plan. For example, if I make a significant plan change regarding cost, and consequently need to update my Cost Management Plan, the overall Project Management Plan will need an update as well. This is most often seen with the "Big 3": cost, scope, and schedule.
- Steven